Characteristics and Classifications of Prochordata
The organisms belonging to the Protochordata are generally known as the lower chordates.
They don’t form a “proper” taxonomic group and are only classified as such for convenience
purposes. However, they do form a major division of Chordata. They are also known as
Acraniata because they lack a true skull. They are divided into three sub-phyla- Hemichordata,
Urochordata, Cephalochordata.
2. Their body is bilaterally symmetrical, triploblastic, and coelomated.
3. At a certain stage of their lives, their body develops a long, rod-like structure for
support called the notochord.
E.g., Herdmania, Amphioxus.
Classifications of Protochordata
Some live solitarily, and some stay in colonies.
The body is cylindrical, unsegmented, and stout.
The body is divided into proboscis, collar, and trunk.
The collar bears arms and tentacles.
They have a complete digestive system.
They respire through gills or general body surface.
The circulatory system comprises a heart with two longitudinal vessels.
The blood has no colour and corpuscles.
The proboscis gland or glomerulus make up the excretory system.
Sexes may be separate or united and fertilization is either internal or external.
E.g., Cephalodiscus, Rhabdopeura.
They are sessile and filter-feeders.
They are also known as tunicates because their body is surrounded by a leathery sheath composed of tunicin (cellulose).
The notochord appears in the larval stage in the tail of the larva and disappears in the adult. This is known as retrogressive metamorphosis.
Respiration occurs through gills.
The excretory organs are absent.
They reproduce asexually by budding.
E.g., Herdmania, Selpa
They are marine and filter-feeders.
The notochords remain throughout life and extend up to the head region.
The nerve cord and the tail also remain throughout life.
Solenocyts are the excretory organs.
They respire through gills which open in the atrium.
The body wall comprises myotomes.
E.g., Amphioxus
What are Protochordates?
Protochordate is an informal category of organisms to describe the invertebrates that are closely related to vertebrates.
What are the sub-phyla of Protochordata?
Protochordata is divided into the following three sub-phyla:
Hemichordata
Urochordata
Cephalochordata
What is the difference between Protochordates and Chordates?
The Chordates are identified by the presence of a notochord. On the contrary, the Protochordates lack a true notochord.
A notochord is the primitive beginning of the backbone found in the embryonic stage. These are only found in the organisms belonging to phylum Chordata. Humans
belong to this phylum and possess a notochord at their embryonic stage.
What is the difference between a notochord and a vertebral column?
A notochord arises from the mesoderm and protects the body of the chordates at the
embryonic stage. Whereas, a vertebral column extends from the neck to tail and protects the
backbone in adult chordates.
Agnatha "without jaws") is a superclass of jawless fish in the phylum Chordata,
subphylum Vertebrata, consisting of both present (cyclostomes) and extinct
(conodonts and ostracoderms) species. The group is sister to all vertebrates with jaws, known
as gnathostomes.
data[8] strongly supports the hypothesis that living agnathans, the cyclostomes,
are monophyletic.[9]
The oldest fossil agnathans appeared in the Cambrian, and two groups still survive today:
the lampreys and the hagfish, comprising about 120 species in total. Hagfish are considered
members of the subphylum Vertebrata, because they secondarily lost vertebrae; before this
event was inferred from molecular[6][7][10] and developmental[11] data, the group Craniata was
created by Linnaeus (and is still sometimes used as a strictly morphological descriptor) to
reference hagfish plus vertebrates. In addition to the absence of jaws, modern agnathans are
characterised by absence of paired fins; the presence of a notochord both in larvae and adults;
and seven or more paired gill pouches. Lampreys have a light sensitive pineal
eye (homologous to the pineal gland in mammals). All living and most extinct Agnatha do not
have an identifiable stomach or any appendages. Fertilization and development are both
external. There is no parental care in the Agnatha class. The Agnatha are ectothermic or cold
blooded, with a cartilaginous skeleton, and the heart contains 2 chambers.
While a few scientists still regard the living agnathans as only superficially similar, and argue
that many of these similarities are probably shared basal characteristics of ancient vertebrates,
recent classification clearly place hagfish (the Myxini or Hyperotreti) with
the lampreys (Hyperoartii) as being more closely related to each other than either is to the
jawed fishes.
Agnathans are jawless fish. Gnathostomata are fish that have jaws. Both agnathans and
Gnathostomata are very important in determining evolutionary relationships
Pisces, a class of vertebrates comprising the true fishes, have the jaws supported by a
skeleton derived from primitive gill arches. Typically, they have two sets of paired fins,
pectoral and pelvic, as well as dorsal, caudal, and anal fins in the midline.
The Superclass Pisces (L. Piscis = fish) are the truly jawed vertebrates. They have
organs of respiration and locomotion related to a permanently aquatic life. The respiratory
organs are the gills and the organs of locomotion are paired and impaired fins.
The super class Pisces (L., Piscis, fish) includes all the fishes which are essentially
aquatic forms with paired fins for swimming and gills for locomotion. About 40,000 species
of fishes are known. Various workers have provided different schemes of their classification.
However, no classification had been universally accepted because of the confusion due to
staggering numbers of fishes and great diversity in their shape, size, habits and habitat.
According to Parker and Haswell (1960) the super class Pisces if sub-divided into three classes.
CLASS-I-PLACODERMI
(i) These include all extinct fishes found from early Devonian to Permian period. (ii) Body
heavily armoured with bony scales or plates. (iii) Primitive jaws with (iv) Paired or unpaired
fins were present. (v) Skeleton bony. (vi) Notochord persistent throughout life. (vii) They are
immediate ancestors of astracoderms he class placodermi includes six subclasses. (i)
Acanthodii, Ex: Climatius (ii) Arthrodin, Ex: Cocoosteus (iii) Petalichthyda, Ex: Macropetalch
(iv) Antiarchi, Ex: Pterich thyodes (v) Rhenanida, Ex: Gemuendina (vi) SPalaeospondylia,
Ex: Palaeospondylus
(ii) Body fusiform or spindle shaped. (iii) Endeskeleton is cartilaginous (iv) Skin with placoid
scales. (v) Fins both median and paired all, supported by fin rays. Pelvic fin bears claspers in
males. (vi) Notochord tail fin heterocereal persistant. Vertebrae complete and separate from
notochord. (vii) Mouth ventral in position, jaws p[resent. Teeth are modified placoid scales.
(viii) Digestive system complete, stomach J-shaped, intestine with spiral valves. (ix) Gill slits
separate, 5-7 pairs, laterally placed, without operculum. (x) Air bladder and lungs absent. (xi)
Heart 2-chambered, poikilothermous. (xii) Kidneys opisthonephrii, Ureotelic. (xiii) Sexes
separate, Gonads paired, fertilization internal, oviparous or ovoviviparous. It includes two sub-
classes.
SELACHII (Elasmorbranchii (i) Multiple gill slits on either side protected but individual skin
flaps. (ii) A spiracle behind each eye.(iii) Cloaca present.Ex: Scolodon (shark), Torpedo
(Electric ray) pristis (saw fish)
(ii) HOLOCEPHALUI (i) Single gill opening on either side covered by a fleshy operculum.
(ii) No spiracles, cloaca and sales. (iii) Single nasal opening. Ex: Chimaeras or Rat fishes.
OSTEICHTHYES (Teleostomi) (i) Body is spindle shaped. (ii) Median and paired fins are
present.(iii) Tail fin is usually Homocercal. (iv) Endoskeleton is partly or wholly bony. (v) Skin
covered by 3 types of dermal scales ganoid, cycloid or ctenoid. Some without scales. No
placoid scales. (vi) Mouth terminal or subterminal Jaws usually with teeth. Cloaca lacking,
anus present. (vii) Gills are covered by a common operculum on either side. (viii) An air (swim)
bladder often present with or without duct connected t pharynx. (ix) Adult kidneys
mesonephric. (x) Well developed lateral line system. Internal ear with 3 semicircular canals.
(xi) Sexes separate, Gonads paired fertilization usually external.(xii) Mostly oviparous, rarely
ovoviviparous or viviparous. This subclass includes two sub-classes.
Sarcopterygii (i) Paired fins are leg-like or lobed with a fleshy, bony central axis covered by scales. (ii)
Internal nares present. (iii) All are fresh water forms. Ex: – Latimeria (living fossil) Protopterus
& Lepidosiren (Lung-fishes) Actinoptergii (i) Paired fins thin, broad without fleshy basal
lobes. (ii) Double external nares are present (iii) Popularly called ray-finned fish. This subclass
is divided into the infraclasses or super orders (i) Chondrostei Ex: – Polypterus (ii) Holostei
Ex: – Lepidosteus (iii) Telestoei Ex: – Labeo, Catla
Balanoglossus: Habitat, Development and Affinities
Balanoglossus is a burrowing and exclusively marine animal. It is found in shallow waters between tide marks along the coast of warm and temperate oceans. Balanoglossus is world-wide in distribution. Balanoglossus is tubicolous living in U- shaped burrows excavated in the sandy bottom.
External Morphology of Balanglossus:
The body of Balanoglossus is soft, elongated, cylindrical, being richly ciliated all over and
covered with mucus.
Proboscis
The proboscis forms the anterior part of the body and is either rounded or conical in
shape. It is continued posteriorly into a short, narrow neck or proboscis stalk.
Collar
The collar lies posterior to the proboscis and anterior to the trunk. It is a short cylinder
usually about as wide as long and mostly shorter than the proboscis although sometimes
longer.
Trunk:
The trunk is the elongated posterior part of the body. It is somewhat flat and annulated
on the surface. It has a mid-dorsal and a mid-ventral longitudinal ridge. The trunk is
divisible into three parts, an anterior branchio-genital region, a middle hepatic region,
and a posterior abdominal or post-hepatic region.
Body Wall of Balanoglossus The body wall of Balanoglossus is made up of an outer epidermis and an inner musculature.
It consists of a single layer of epithelial cells. The epithelial cells are of tall columnar type
and have their nuclei near their broader bases.
These cells are mainly of two types:
(i) Ciliated epidermal cells are more numerous and each bears cilia at its free end;
(ii) Gland cells are lying interspersed between the ciliated epidermal cells and are further of
three kinds
(a) Goblet cells are flask-shaped and secrete mucus;
(b) Reticulate cells are long cells with vacuolated cytoplasm which also secrete mucus;
Musculature:
The musculature of typical body-wall and gut-wall is greatly reduced and more or less
replaced by muscles arising from the coelomic epithelium. The muscle fibres are
smooth and of circular, longitudinal and diagonal types. The muscle layer lies below
the basement membrane.
Coelom in Balanoglossus:
The coelom is enterocoelous having been formed as outgrowths of the enteron.
Corresponding with the three body regions the coelom is divided into three portions
which are completely separated from each other by septa. The coelom is lined with
coelomic epithelium or peritoneum.
Proboscis Coelom:
The proboscis coelom or protocoel is a single space in the proboscis which is largely
occupied by muscles and connective tissue and a few structures like buccal
diverticulum, glomerulus and central sinus or heart.
Collar Coelom:
The collar coelom or mesocoel has two cavities lying side by side in the collar, one on each
side between the collar wall and buccal cavity. The two cavities are partitioned by incomplete
mid-dorsal and mid-ventral mesenteries.
Trunk Coelom:
The trunk coelom or metacoel has two closed cavities lying between the body wall and
alimentary canal. The two cavities are separated by an incomplete dorsal and a
complete ventral mesentery.
Digestive System of Balanoglossus:
Alimentary Canal of Balangolossus:
In Balanoglossus, the alimentary canal is a straight tube. Its anterior opening, the mouth, is wide and circular and situated on the ventral side in a groove between the proboscis stalk and collarette. The mouth remains open constantly.
The posterior opening or the anus is a circular aperture at the extreme posterior end of the trunk. Between the mouth and anus, the alimentary canal can be distinguished into four regions buccal tube, pharynx, oesophagus, and intestine. Their walls are composed of
ciliated epithelium lined externally by basement membrane and devoid of muscle fibres.
1. Buccal Tube
The mouth leads into a buccal tube or cavity in the collar region. Its epithelial
wall contains glandular goblet cells. The dorsal wall of buccal tube forms a short, stiff
and hollow buccal diverticulum that projects into the proboscis coelom.
Pharynx:
The wall of the roof of the buccal tube opens into the pharynx lying in the branchial
region of the trunk. Its wall bears a longitudinal constriction along each lateral side.
Branchial Portion of Pharynx:
The dorsal branchial portion of pharynx is perforated dorsolaterally by two rows of U-
shaped gill-slits. It is concerned with respiration.
(ii) Digestive Portion of Pharynx:
The digestive portion of pharynx is concerned with the food concentration, digestion
and absorption of food. Its ciliated epithelial wall contains gland cells.
3. Oesophagus:
Behind the last pair of gill-slits the pharynx continues into the oesophagus. The dorsal
and ventral divisions of the pharynx continue for some distance into the oesophagus;
in this region, the dorsal part of the oesophagus is called post-branchial canal which
possesses thick, folded and glandular epithelium.
Behind the oesophagus is an intestine, It occupies the hepatic and post-hepatic regions
of the trunk. The hepatic region of the intestine is highly vascular. Its epithelial cells
are dark green or dark brown and its dorsal wall forms numerous prominent
sacculations called hepatic caeca which push the body wall outwards and are, thus,
visible externally.
Food, Feeding and Digestion of Balangolossus:
Balanoglossus is a “ciliary feeder”. Its food comprises microscopic organisms and
minute organic particles present in water and mud or bottom sand in which it makes
its burrows. The lateral cilia lining the gill-slits set up a current of water directed
backward which enters through the mouth, takes its course through the buccal tube,
pharynx, gill-slits and branchial sacs, and leaves through gill-pores.
Respiratory System of Balanoglossus:
(1) The branchial portion of pharynx bearing gill-slits
(2) The branchial sacs that open out through gill-pores
1. Branchial Pharynx:
As already described, two lateral longitudinal parabranchial ridges divide the pharyngeal cavity
into a dorsal respiratory or branchial portion and a ventral digestive portion. Dorsolaterally, on
each side, the branchial portion of pharynx is perforated by a longitudinal series of numerous
U-shaped openings, the gill- slits.
Mechanism of Respiration:
The lateral cilia lining the gill-slits create a current of water (food-cum-respiratory
current) that enters the pharynx through mouth, then passes through gill-slits into the
branchial sacs and finally leaves through the gill-pores. The tongue bars are richly
supplied with blood capillaries and take part in respiration. The blood of their capillary
networks takes up oxygen dissolved in water and diffuses carbon dioxide to it.
Excretory System of Balanoglossus:
In Balanoglossus, the excretory organ is glomerulus or proboscis gland lying in front
of the central sinus and projecting into the proboscis coelom. The glomerulus is made
up of several blind tubular projections formed by the peritoneum covering the buccal
diverticulum, central sinus and heart vesicle. The tubular projections of glomerulus
are filled with blood which is confluent with the blood of the central sinus.
Reproductive System of Balanoglossus:
In Balanoglossus, the sexes are separate and are indistinguishable externally except in
case of the colour of the ripe gonads shown through the body wall in the living animal.
The gonads occur in one or more longitudinal rows to the sides of the alimentary canal
lying within the genital pleurae in the anterior part of the trunk. Gonads develop from
the coelomic wall, though they have no connection with coelom in the adult.
Asexual Reproduction
Asexual reproduction is known to occur in Balanoglossus capensis (Gilchrist, 1923).
During summer the juvenile phase of this, at first considered a distinct species for it
lacks hepatic sacculations,
Affinities of Balanoglossus:
The position of Hemichordata, in the scheme of classification of animals, has been
controversial. In 1814, Sedgwick and Huxley suggested the affinities of Enteropneusta
(Hemichordata) with the vertebrates and it was in 1885 Bateson considered this group
as a subphylum of the phylum Chordata.
Metschnikoff (1865) stated that Enteropneusta had certain affinities with
Echinodermata. Spengel (1893) showed the relationship of Enteropneusta with
Annelida.
But on the basis of general organisation, some recent workers, such as Van der Horst
(1939), Dawydoff (1948), Marcus (1958) and Hyman (1959) have thought it proper to
remove this group from phylum Chordata to give it the status of an independent
invertebrate phylum.
The name “Hemichordata” is, however, retained for the group because it suggests
that its members are related to chordates, i.e., they are “half’ or “part” chordates, a
fact that is undisputed.
Scoliodon Detailed Study
Introduction to Scoliodon:
The class Elasmobranchii embraces a large variety of cartilaginous fishes. There are
several genera of dogfishes available in the different parts of the earth. The typical
Indian genus, Scoliodon is described below. The genus is represented by about nine
species, of which four are very common in the Indian seas.
The genus Scoliodon is distinguished from other dogfishes by having an elongated
snout, depressed head and a compressed body. The teeth are similar in both the jaws.
The caudal pit and the sub-caudal lobe are prominent and distinct. The four Indian
species of Scoliodon are: Scoliodon sorrakowah, S. dumerilii, S. palasorrah and S.
walbeehmi.
Habit and Habitat of Scoliodon:
The shark is a marine, carnivorous and predaceous animal. It eats small pelagic
schooling and bottom living bony fishes, including anchovies, codlet (Bregmacero-
tidae), burrowing gobies (Tripauchenidae) and Bombay ducks (Harpadontidae) as
well as shrimps and cuttle fish.
Both sexes mature between 1-2 years old and the males reach largest size at the age of
about 5 years and females reach largest size at the age of 6 years.
Geographical Distribution of Scoliodon:
Scoliodon has a wide geographical distribution ranging from Zanzibar to Ceylon (Sri
Lanka), Ceylon to the Malay Archipelago of the Indian Ocean, Bay of Bengal, Eastern
Pacific , West Indies and eastern coasts of South America. Fossils of Scoliodon have
been discovered in the geological strata from the lower Eocene to later periods.
External Structures of Scoliodon:
Scoliodon is an elongated spindle-shaped animal. It has a laterally compressed body. A fully-
developed specimen of the genus attains a length of about 60 cm. The body is divisible into
head, trunk and tail. The head is dorsoventrally flattened and terminates anteriorly into a
dorsoventrally compressed snout. The dorsal side of Scoliodon is dark-grey while the underside
is pale white.
The trunk is more or less oval in transverse section. It attains maximum thickness in the middle
region and the body gradually tapers posteriorly into a long tail
The mouth is a very wide crescentic aperture lying on the ventral side of the head
near its anterior end. The teeth are replaced if these are broken. The teeth of Scliodon are
modified scales. Two prominent circular eyes are present. Each eye is provided with movable
upper and lower eyelids. The nostrils are placed one at each angle of the mouth. These are
exclusively olfactory in function and have no connection with the mouth cavity. The cloaca
opens to the exterior by a cloacal aperture which is located in between the two pelvic fins. The
abdominal pores are paired structures and situated on elevated papillae to communicate the
coelom to the outside.
As in other fishes, Scoliodon bears unpaired and paired fins which are actually flap-
like integumentary extensions of the body.
Median unpaired fins:
The fins under this category include two dorsals, one caudal and one ventral fin. The
dorsal fins are triangular in outline. The anterior dorsal is larger and situated at about
the middle of the body.
Lateral paired fins:
Two pectoral and two pelvic fins constitute the lateral paired fins. The pectoral fins are
large and are situated posterior to the gill-clefts. The pelvic fins are much smaller.
Skin of Scoliodon: The integument is composed of an outer epidermis and an inner dermis. The epider- mis is composed of epithelial cells intermixed with numerous unicellular mucous glands. In the young stage the epithelial cells are ciliated. But in an adult the cilia are lost. The dermis is composed of three layers: (i) Stratum laxum, (ii) Stratum compactum and (iii) Subcutaneous layer.
Locomotion in Scoliodon:
The movement of Scoliodon is caused by the activities of the myotomal longitudinal muscle
fibres and is also aided by movement of the fins. In the phylogenetic history of the fishes, the
fins were primarily employed to raise the body off the bottom, but these become secondarily
used in swimming by producing undulatory movements.
The longitudinal muscle fibres composing the myotomes play the important role in swimming.
The myotomes are placed on either side of the incompressible vertebral column which acts as
a lever upon which the myotomes work.
Coelom of Scoliodon: In Scoliodon, the coelom is spacious and is divided into a smallest pericardial cavity and an
extensive abdominal cavity. These two cavities are separated by the septum transversum and
communicate
Digestive System of Scoliodon:
The digestive system consists of the alimentary canal and the digestive glands. The alimentary
canal starts with the mouth and terminates in the anus. The mouth leads into a spacious buccal
cavity which is lined with mucous membrane.
There are two types of gills: (i) Holobranch or complete gill when a branchial arch bears two sets of gill lamellae and (ii)
Demi branch or hemi branch or half gill when single set of gill lamellae is present. The hyoid
arch supports only a demi branch and the first four branchial arches support holobranchs. The
last branchial arch is gill-less. Mechanism of respiration: During respiration the floor of the
buccal cavity is lowered and the mouth is opened. Then the water rushes in to fill the greatly
expanded buccal cavity. The mouth is now closed and the pharynx contracts.
The water then enters the gill-pouches and goes out after gaseous exchange through gill-slits.
The spiracles are occasionally used as accessory pathways for the entry of water for respiration
instead of the mouth when it is otherwise occupied. Circulatory System of Scoliodon: The
circulatory system consists of: (a) The circulatory fluid, called blood, (b) The heart, (c) The
arteries and (d) The veins. Blood: The blood consists of a colourless plasma and corpuscles
are suspended in the plasma. Two kinds of corpuscles are encountered; the RBC (or
erythrocytes) and the WBC (or leucocytes). Heart: The heart is a bent muscular tube and
consists of the receiving parts, comprising of a sinus venosus and a dorsally placed auricle, and
the forwarding parts, consisting of a ventricle and a conus arteriosus . The heart is situated on
the ventral side of the body between two series of gill-pouches. Receiving parts of the heart:
The sinus venosus is a thin-walled tubular chamber. The sinus venosus is highly contractile
and the beating of the heart originates from this part of the heart. Two great veins, the ductus
Cuveiri, open into the sinus venosus, one on each lateral side. Two hepatic sinuses enter the
sinus venosus posteriorly. The sinus venosus opens into the auricle by sinuauricular aperture
which is guarded by a pair of valves. The auricle is a large, triangular and thin-walled chamber
situated dorsal to the ventricle but in front of the sinus venosus. The auricle communicates with
the ventricle through a slit-like auriculoventricular aperture guarded by two lipped
valves. The receiving chambers, (sinus venosus and auricle) receive the venous blood
from all parts of the body.
The venous system is extremely complicated and is described under the following heads:
(A) Cardinal system:
The blood from the anterior region of the body is returned to the heart by paired jugular and
anterior cardinal sinuses. The blood from the posterior region is collected by a pair of posterior
cardinal sinuses. The anterior and posterior cardinals unite on each side to form a transverse
sinus called ductus Cuvieri.
(i) Anterior cardinal system:This system of veins returns blood from the head region and
consist of a pair of internal jugular veins. Each internal jugular vein is composed of the
olfactory sinus, the orbital sinus, the postorbital sinus and the anterior cardinal sinus. The blood
from the rostral region is drained by the anterior facial vein to the olfactory sinus and from
there to the orbital sinus.
The orbital sinus opens into the anterior cardinal sinus through the postorbital sinus. The
anterior cardinal sinus enters the ductus Cuvieri. The anterior cardinal sinus receives the
hyoidean sinus and five dorsal nutrient branchial sinuses from the gills.
(ii) Posterior cardinal system: The caudal vein collects blood from the tail region and
proceeds forwards through the haemal canal. In the abdominal cavity, the caudal vein divides
into left and right renal portal veins which break up into sinusoid capillaries in the substance
of the kidneys.
(B) Hepatic portal system:
A large number of small veins carrying blood from the alimentary canal and its associated
glands unite to form the hepatic portal vein. The hepatic portal vein receives the lienogastric
vein and anterior and posterior gastric veins.
C) Cutaneous system: This system consists of a dorsal, a ventral and two paired lateral
cutaneous veins. The inferior lateral cutaneous vein joins with the lateral cutaneous vein near
the anterior end of the pectoral fin. Each lateral cutaneous vein ultimately opens into the
brachial vein.
(D) Ventral system:
This system comprises of two sets of veins—the anterior ventral veins pouring blood to the
ductus Cuvieri through inferior jugular sinuses and the posterior veins which discharge through
the subclavian vein.
The nervous system of Scoliodon includes:
(i) The central nervous system, (ii) The peripheral nervous system and (iii) The autonomous
nervous system. Central nervous system: The central nervous system consists of brain and
the spinal cord. Brain: The brain is highly organised and shows many advancements over
that of the agnathans. The brain is divided into three primary parts: (a) The forebrain or
prosencephalon, (b) The midbrain or mesencephalon and (c) The hindbrain or
rhombencephalon. The forebrain consists of a massive undivided cerebral hemisphere. The
cerebral hemisphere is relatively larger than that of other fishes. From the anterior end of
cerebral hemisphere arise two stout olfactory peducles, each terminates into a large bilobed
olfactory lobe . The olfactory lobes lie close to the olfactory capsules. Each olfactory nerve is
composed of many bundles of nerve fibres. The surface of the cerebrum is smooth and the
walls are thick.
Peripheral nervous system:
The peripheral nervous system includes the cranial nerves and spinal nerves. Cranial nerves:
There are ten pairs of cranial nerves in all the fishes. An extra pair of anterior termin The fifth
cranial nerve is the trigeminal which has three branches: (a) Ophthalmicus superficialis
which supplies the skin of the snout;(b) The maxillaris which is divided into maxillaris superior
supplying nerves to the skin of the upper jaw and maxillaris inferior innervating the posterior
part of the upper jaw al or pre-olfactory nerves is present in Scoliodon.
The seventh cranial nerve is known as facial which divides into two branches The
branches are: The brachial nerves supplying the gills, (ii) The lateralis supplying the lateral
line sense organs and gives numerous branches along its course. Each spinal nerve gives
three branches, such as: a) Ramus dorsalis,(b) Ramus ventralis and (c) Ramus communicans
to join with the autonomous nervous system.
These eye muscles are attached with the eye ball in two groups. The first group include:(i)
Superior rectus, (ii) Inferior rectus, (iii) Anterior rectus and (iv) Posterior rectus. The second
group include: (i) Superior oblique and ii) Inferior oblique.
Urinogenital System of Scoliodon: The excretory organ consists of a pair of elongated kidneys. The functional adult kidneys are
called opisthonephros type according to Graham Kerr. The anterior portion of the kidney is
non-functional and the po terior portion becomes greatly developed.
Male reproductive system: The testes are paired elongated organs (Fig. 6.17A). Each testis
is attached to the dorsal body wall by peritoneal membrane called mesorchium and posteriorly
attached by ordinary tissue with the caecal gland. The sperm cells escape by vasa efferentia
into the vas deferens which becomes extremely coiled in the anterior portion of the kidney.
Female reproductive system:
In females there is no connection between the kidneys and the genital organs. The kidneys are
typical excepting that the ureters unite posteriorly and open by a single urinary aperture into
the urinary sinus. The ovaries are two in number and are kept in position by peritoneal folds
called mesovarium.
Accessory Respiratory Organs in Fishes | Phylum Chordata Types of Accessory Respiratory Organs: 1.
Suprabrachial Organ: The supra-branchial organ is a specialised type of respiratory structure
encountered in Clarias batrachus . It has a complex structural organisation and consists of the
following portions: (a) An elaborate tree-like structure growing from the upper end of the second and
fourth gill-arches of either side. This dendritic organ is composed of numerous terminal knobs, each
has a core of cartilage covered by vascular membrane. Each exhibits eight folds which suggest that
one such knob is formed by the coalescence of eight gill-filaments.
(b) There are a pair of highly vascularized supra- branchial chambers within which the tree-like
structures are contained. The supra-branchial chambers are developed as the vascularized diverticula
of the branchial chamber. (c) The entrance of the supra-branchial chamber is guarded by ‘fan’-like
structures which are developed by the fusion of the adjacent gill- filaments of the dorsal side of the
gill-arches. The supra-branchial organs, like the gills, are lined by thin outer epithelial layers with
intercellular spaces separated by the pilaster cells. These fishes come to the surface of the water and
gulp air into the supra-branchial organs. Atmospheric air from the pharyngeal cavity is taken into the
supra-branchial chamber by an inhalant aperture located between the second and third gill- arches.
After gaseous exchange the air from the said chamber expels into the opercular cavity by the gill-slit
lying between the third and fourth gill-arches. The fan-like structures present in the second and the
third gill-arches help to intake the air while the expulsion of the air from the supra-branchial, chamber
is caused by the contraction of its wall. Thus the supra-branchial chamber and its contained organs
function as ‘lung’.
2. Branchial Outgrowths: In climbing perch (Anabas testudineus) there are two spacious sac-like
outgrowths from the dorsal side of the branchial chambers The epithelium lining these outgrowths is
highly vascular and becomes folded to increase the respiratory area. Each chamber contains a
characteristic rosette-like labyrinthine organ. This organ develops from the first epibranchial bone and
consists of a number of shell like concentric plates. The margins of the plates are wavy and the plates
are covered with vascular gill-like epithelium. Each branchial outgrowth communicates freely not only
with the opercular cavity but also with the buccopharyngeal cavity. Air enters into the outgrowth by
way of the buccopharyngeal opening and goes out through the external gill-slits. The entrance is
controlled by valves. During travelling the opercula alternately spread out and fix to the ground by the
spines and get the forward push from the pectoral fins and the tail. The proverb that the fish can climb
the trees seems to be erroneous. The climbing perches are found in the branches of palm or other
trees which are possibly brought there by the kites or crows while these fishes migrate over the land.
In Trichogaster fasciatus the accessory respiratory organs are similar to that of Anabas and consist of
supra-branchial chamber, labyrinthine organ and respiratory membrane Each of these two expansions
is composed of loose connective tissue which is covered by highly vascular epithelium. 3. Pharyngeal
Diverticula: Anabas can breathe in air by the help of these organs. These fishes have the habit of
migration from one pond to the other. Their overland progression is peculiar and is assisted by the
operculum and the fins. Each operculum bears sharp spines at the free edge. In the Snake- headed
fishes and Cuchia eels, the accessory respiratory organs are relatively simplified. These fishes can
survive prolonged drought and their air breathing habit enables them to remain out of water for some
time. In both the group of fishes, the pharynx gives a pair of saclike diverticula for gaseous exchange.
Accessory Respiratory Organs in Fishes | Phylum Chordata Types of Accessory Respiratory Organs: 1.
Suprabrachial Organ: The supra-branchial organ is a specialised type of respiratory structure
encountered in Clarias batrachus It has a complex structural organisation and consists of the following
portions: (a) An elaborate tree-like structure growing from the upper end of the second and fourth
gill-arches of either side. This dendritic organ is composed of numerous terminal knobs, each has a
core of cartilage covered by vascular membrane. Each exhibits eight folds which suggest that one such
knob is formed by the coalescence of eight gill-filaments.
(b) There are a pair of highly vascularized supra- branchial chambers within which the tree-like
structures are contained. The supra-branchial chambers are developed as the vascularized diverticula
of the branchial chamber. (c) The entrance of the supra-branchial chamber is guarded by ‘fan’-like
structures which are developed by the fusion of the adjacent gill- filaments of the dorsal side of the
gill-arches. The supra-branchial organs, like the gills, are lined by thin outer epithelial layers with
intercellular spaces separated by the pilaster cells. The organs and the supra-branchial chambers are
supplied by afferent and efferent blood vessels from the gill-arches. The supra-branchial organs help
to breathe in air. The supra-branchial chamber has inhalant and exhalant apertures. These fishes come
to the surface of the water and gulp air into the supra-branchial organs. Atmospheric air from the
pharyngeal cavity is taken into the supra-branchial chamber by an inhalant aperture located between
the second and third gill- arches. After gaseous exchange the air from the said chamber expels into
the opercular cavity by the gill-slit lying between the third and fourth gill-arches. The fan-like
structures present in the second and the third gill-arches help to intake the air while the expulsion of
the air from the supra-branchial, chamber is caused by the contraction of its wall. Thus the supra-
branchial chamber and its contained organs function as ‘lung’. 2. Branchial Outgrowths: In climbing
perch (Anabas testudineus) there are two spacious sac-like outgrowths from the dorsal side of the
branchial chambers. The epithelium lining these outgrowths is highly vascular and becomes folded to
increase the respiratory area. Each chamber contains a characteristic rosette-like labyrinthine organ.
This organ develops from the first epibranchial bone and consists of a number of shell like concentric
plates. The margins of the plates are wavy and the plates are covered with vascular gill-like epithelium.
Each branchial outgrowth communicates freely not only with the opercular cavity but also with the
buccopharyngeal cavity. Air enters into the outgrowth by way of the buccopharyngeal opening and
goes out through the external gill-slits. The entrance is controlled by valves. During travelling the
opercula alternately spread out and fix to the ground by the spines and get the forward push from the
pectoral fins and the tail. The proverb that the fish can climb the trees seems to be erroneous. The
climbing perches are found in the branches of palm or other trees which are possibly brought there
by the kites or crows while these fishes migrate over the land. In Trichogaster fasciatus the accessory
respiratory organs are similar to that of Anabas and consist of supra-branchial chamber, labyrinthine
organ and respiratory membrane 3. Pharyngeal Diverticula: Anabas can breathe in air by the help of
these organs. These fishes have the habit of migration from one pond to the other. Their overland
progression is peculiar and is assisted by the operculum and the fins. Each operculum bears sharp
spines at the free edge. In the Snake- headed fishes and Cuchia eels, the accessory respiratory organs
are relatively simplified. These fishes can survive prolonged drought and their air breathing habit
enables them to remain out of water for some time. In both the group of fishes, the pharynx gives a
pair of saclike diverticula for gaseous exchange. In Channa, the accessory respiratory organs are
relatively simpler and consist of a pair of air-chambers (Fig. 6.83D). These are developed from the
pharynx and not from the branchial chamber as seen in others. The air-chambers are lined by
thickened epithelium which is highly vascularized. The air-chambers are simple sac-like structures and
do not contain any structure. These chambers function as the lung-like reservoirs. In Channa striatus
the vascular epithelium lining the chambers becomes folded to form some alveoli. The gill-filaments
are greatly reduced in size. In Cuchia These diverticula open anteriorly into the first gill-slit. These
diverticula function physiologically as the lungs. The gills are greatly reduced and a few rudimentary
gill-filaments are present on the second of the three remaining gill- arches. The third gill-arch is found
to bear fleshy vascular epithelium. In Periophthalmus, a pair of very small pharyngeal diverticula is
present which are lined by vascular epithelium. 4. Pneumatic Sacs: In Heteropneustes fossil is, a pair
of tubular pneumatic sacs, one on each side of the body, act as the accessory respiratory organs. These
long tubular sacs arise as the outgrowths from the branchial chamber and extend almost up to the tail
between the body musculature near the vertebral column (Fig. 6.83C). In Sacco-branchus, similar
tubular lung-like outgrowths of the branchial chamber extend back into the body musculature. 5.
Buccopharyngeal Epithelium: The vascular membrane of buccopharyngeal region in almost all the
fishes helps in absorbing oxygen from water. But in mudskippers (Periophthalmus and
Boleophthalmus) the highly vascularized buccopharyngeal epithelium helps in absorbing oxygen
directly from the atmosphere. These tropical fishes leave water and spend most of the time skipping
or ‘walking’ about through dampy areas particularly round the roots of the mangrove trees. The old
idea that the mud-skippers use the vascular tail as the respiratory organ is not supported by recent
Icthyologists. 6. Integument: Eels are recorded to make considerable journey through damp
vegetation. The common eel, Anguilla Anguilla can respire through the integument both in air and in
water. In Amphipnous cuchia and mud- skippers, the moist skin sub-serves respiration. Many embryos
and larvae of fishes respire through the skin before the emergence of the gills. The median fin fold of
many larval fishes is supplied with numerous blood vessels and helps in breathing. The highly vascular
opercular fold of Sturgeon and many Catfishes serves as the accessory respiratory structure. 7. Gut
epithelium: The inner epithelium of the gut essentially helps in digestive process. But in many fishes
the gut becomes modified to sub-serve respiratory function. Cobitis (giant loach of Europe) comes
above the water-level and swallows a certain volume of air which passes back along the stomach and
intestine. In Misgurus fossilis, a bulge just behind the stomach is produced which is lined by fine blood
vessels. The bulge acts as the reservoir of air and functions as the accessory respiratory organ. After
the gaseous exchange, the gas is voided through the anus. In certain other fishes, Callichthyes,
Hypostomus and Doras the highly vascular rectum acts as the respiratory organ by sucking in and
giving out water through the anus alternately. In these fishes the wall of the gut becomes modified.
The wall becomes thin due to the reduction of the muscular layers. 8. Swim-Bladder acts as Lung:
Swim-bladder is essentially a hydrostatic organ but in some fishes it functions as the ‘lung’. In Amia
and Lepisosteus, the wall of the swim-bladder is sacculated and resembles lung. In Polypterus the
swim-bladder is more lung-like and gets a pair of pulmonary arteries arising from the last pair of
epibranchial arteries. The swim-bladder in dipnoans resembles strikingly the tetra- pod lung in
structure as well as in function. In Neoceratodus, it is single, but in Protopterus and Lepidosiren it is
bilobed. The inner surface of the ‘lung’ is increased by spongy alveolar structures. In these fishes, the
‘lung’ is mainly respiratory in function during aestivation because the gills become useless during this
period. Like that of Polypterus, the ‘lung’ in dipnoans gets the pulmonary arteries from the last
epibranchial arteries. In Notopterus, the swim-bladder becomes more complex and acts as a lung.
Except the hydrostatic, sound production and hearing, a new function like respiration was innovated
in Notopterus. In Notopterus chitala the posterior tip of swim-bladder is enlarged which is called
caudal extension and the ventral part gives off several fingerlike projections, the dorsal side of the gas
bladder possesses a specialised striated muscle. The anterior part extends into a projection to the ear.
An artery arising from the dorsal aorta forms a network of blood capillaries that spread the entire
inner surface of the abdominal and caecal parts of the swim bladder. The blood capillaries that cover
a single epithelial layer helps in the gaseous exchange between the blood and the air of the swim-
bladder. This air breathing habit is considered as a secondary adaptation in these fishes. Functions of
Accessory Respiratory Organs: The accessory respiratory organs contain a high percentage of oxygen.
The fishes possessing such respiratory organs are capable of living in water where oxygen
concentration is very low. Under this condition these fishes come to the surface of water to gulp in air
for transmission to the accessory respiratory organs. If these fishes are prevented from coming to the
surface, they will die due to asphyxiation for want of oxygen. So the acquisition of accessory
respiratory organs in fishes is an adaptive feature. Further it has been observed that the rate of
absorption of oxygen in such organs is much higher than the rate of elimination of carbon-dioxide.
Hence, it is natural that the gills excrete most of the carbon-dioxide. Absorption of oxygen appears to
be the primary function of the accessory respiratory organs. Significance of Accessory Respiratory
Organs: The cause of emergence of the accessory respiratory structures in fishes in addition to the
primary respiratory organ is very difficult to interpret. There are two contrasting views regarding the
origin of the aerial accessory respiratory structures. First view: some fishes have the natural instinct
to make short excursion to the land from the primal aquatic home. To remain out of water, the
development of certain devices to breathe in air becomes necessary. As a consequence of the air-
breathing habit for a considerable span of time, the fishes have developed specialised accessory
respiratory organs in addition to the gills. Most of such structures encountered in the fishes assume
the shape of reservoir of air and originate either from the pharyngeal or branchial cavities. In extreme
cases the reservoir may house special structure for gaseous exchange. However, the development of
such accessory respiratory organs is essentially adaptive in nature to meet the respiratory need and
thus enables the fishes to tolerate oxygen depletion in water or to live on land over a varying period
of time. The development of the accessory respiratory organs depends directly on the ability to remain
out of the water.
Amphibia: Characteristics, Classification and Examples
The class Amphibia belongs to the subphylum Vertebrata of phylum chordata.
All the representatives of Clssa Amphibia are ectothermic, tetrapod vertebrate animals
which inhabit a wide variety of habitats including terrestrial, arboreal, fossorial, or
freshwater aquatic ecosystems. The name amphibian is derived from Greek word
"amphibious" which means “living a double life”. Because some species are
permanent land dwellers, while other species show an entirely aquatic mode of life.
There are about 8100 known living amphibians, of which nearly 90% are frogs.
General Characteristics Features of the Class Amphibia
1. Amphibians are ectothermic vertebrate animals.
2. They inhabit is a wide variety habitats including terrestrial, arboreal,
fossorial, or freshwater aquatic ecosystems.
3. The body has two parts: head and neck; in some cases, tail and neck may
or may not present.
4. They have soft moist pigmented and glandular skin which does not contain
any scales but the skin of caecilians contains scales. In this case, skin is
smooth and rough and glands keep the skin moist.
5. Mouth is large which contains protusible tongue and homodont type teeth.
6. There are two pairs of limbs which are used for locomotion.
7. The body does not bear paired fins but unpaired fins might be present.
8. They perform respiration through skin and lungs. But larval forms respire
through the gills. In some case, gills might be present externally in the some
adults.
9. Digestive system is complete and the heart is three chambered with two
auricles and one ventricle; portal system is well-developed.
10. The excretory system consists of mesonephric kidney. The excretion is
ureotelic and the excretory waste products are ammonia and urea.
11. Brain is poorly developed with 10 pairs of cranial nerves.
12. The sexes are separate and they usually perform external fertilization. In
case for salamanders, the fertilization is internal.
13. They are egg layers and usually breed in water bodies. The male does not
possess copulatory organs.
14. Indirect development occurs. In this case, tiny larval forms with no legs or
lungs are seen, known as tadpole larva which is transformed into adult
through metamorphosis. In this case most larvae are herbivorous, some
omnivorous or carnivorous.
15. The lateral line is seen during their larval stage.
16. The notochord is present at embryonic stage which does not persist and
transforms into vertebra at adult stage.
Parental care in Amphibia
Parental care refers to any behaviors on the part of either or both parents that help their
offspring survive. In many birds, parental care includes building a nest and feeding the young. ...
For example, tilapia practice a behavior called oral brooding. The mother carries the eggs in her mouth
until they hatch
Parental care is a behavioural and evolutionary strategy adopted by some animals,
involving a parental investment being made to the evolutionary fitness of offspring. Patterns
of parental care are widespread and highly diverse across the animal kingdom.
Care can be beneficial if parents (1) increase offspring survival during the stage in which
parents and offspring are associated, (2) improve offspring quality in a way that leads to increased
offspring survival and/or reproduction in the future when parents are no longer associated with
offspring, and/or (3) directly .
However, it is shown that the adaptation of parental care approaches to the optimal state
when parental care is required for the survival of the population, for example, when the loss rate of
immature or competition among mature increases or the fecundity decreases.
Gymnophiona Structure and biological significance
also called Apoda, one of the three major extant orders of the class Amphibia. Its
members are known as caecilians, a name derived from the Latin word caecus, meaning
“sightless” or “blind.” The majority of this group of limbless, wormlike amphibians live
underground in humid tropical regions throughout the world. Because of their relatively hidden
existence, caecilians are unfamiliar to the layperson and are not usually considered in
discussions about amphibians. They are nevertheless a fascinating group of highly specialized
amphibians about which there is still much to be learned.
No appendicular skeleton - they are completely limbless and have no shoulder girdle, but there is a kink in the spine where the pelvic girdle once was.
95-285 presacral vertebrae (those anterior to the sacral vertebrae, which once fused with the pelvic girdle).
Compound, akinetic skull formed of joined plates of bone - this is an excellent and typical adaptation for a fossorial animal (also seen in burrowing lizards, and burrowing mammals, such as the golden moles), allowing the head to be used like a spade to dig, push, and pack earth when burrowing in underground tunnels.
Reduced eyes.
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Characteristics of Class Amphibia
The characteristics of the organisms present in class amphibia are as follows:
1. These can live both on land and in water.
2. They are ectothermic animals, found in a warm environment.
3. Their body is divided into head and trunk. The tail may or may not be present.
4. The skin is smooth and rough without any scales, but with glands that make it moist.
5. They have no paired fins. Unpaired fins might be present.
6. They have two pairs of limbs for locomotion.
7. They respire through the lungs and skin. Gills might be present externally in some adults.
8. The heart is three chambered.
9. The kidneys are mesonephric. The excretory material includes ammonia and urea.
10. They possess ten pairs of cranial nerves.
11. The lateral line is present during their development.
12. The sexes are separate and fertilization is usually external. However, in salamanders, the
fertilization is internal.
13. Development is indirect with metamorphosis.
14. Breeding occurs in water. The copulatory organs are absent in males.
15. Eg., Frogs, Salamanders.
Classification of Amphibia
The class Amphibia includes all tetrapod vertebrates that are not amniotes. Amphibia in its widest
sense was divided into three subclasses, two of which are extinct:
Subclass: Lepospondyli† (small Paleozoic group, which are more closely related to
amniotes than Lissamphibia)
newts and caecilians)
The Amphibians are divided into three orders. The classification of amphibia is given below:
Apoda (Gymnophiona or Caecilia)
Apoda means “without legs”.These are limbless organisms with scales on their body.
They are also known as “blind-worms” because their eyes are covered by skin or bone.
The tentacles on their head are the chemosensory organs that help them to detect the
underground prey. Eg., Caecilians
They possess venom glands.
Urodela (Caudata)
The skin is smooth with poison glands.
Fertilization is internal.
They feed on insects and worms. Eg., Salamanders
They are found under leaf litter, in the soil, or in water.
In the southern US, they reproduce primarily in winters.
Very little differences between male and female.
Spermatophores are utilized for internal fertilization.
They possess hidden gills.
There are around 3400 species of Anura in the world.
They have four limbs. The front limbs are elongated and modified to jump.
The head and trunk are fused together.
The tail is present only in the larval stage and is lost in the adults.
Fertilization is external and the eggs are laid in water. Eg., frogs and toads.
Frog detail study
Frogs are carnivorous tailless amphibians which are widely found in India. A diverse variety of
frogs can be found all over the world; among them, the Indian frogs are called Rana tigrina. They
are vertebrates, coming under the class Amphibia (phylum Chordata). Frogs are cold-blooded
animals (poikilotherms) whose body temperature varies according to their environment, hence,
they need to protect themselves from extreme heat and cold for maintaining optimum body
temperature. Thus, they follow aestivation and hibernation during the summer and winter seasons
respectively. Another characteristic feature of frogs is that they camouflage i.e., they can change
their skin colour according to their surroundings.
Morphology of Frogs
Though larvae have tails, adult frogs are tailless. An adult frog has a stout body which is
differentiated into head and trunk. Other external features are a pair of nostrils, protruding eyes, a
membranous tympanum (ear), slippery/warty moist skin and webbed limbs.
Frogs generally have a slippery moist and highly permeable skin through which they absorb water
and respire. Thus, the moist skin acts as a respiratory organ in frogs. Also, the skin is glandular in
nature, which produces mucus and toxic substances to warn them of their predators. The colour of
the skin can vary from brown and green to vivid colours as per secretions.
The locomotion of frogs takes place with the help of their forelimbs and hind limbs. Frogs are
unisexual i.e., they show sexual dimorphism. A male frog is distinguished from a female frog by
the presence of vocal sacs and a copulatory pad on forelimbs. A female frog lacks these body
features.
Anatomy
The body plan of frogs consists of well-developed structures which help them in their
physiological activities. The body cavity accommodates all the organ systems such as digestive,
respiratory, circulatory, excretory, nervous and reproductive systems, whose functions are almost
similar to human body systems.
Digestive system
The alimentary canal together with the accessory organs makes up the digestive system of the frog.
Since frogs are carnivorous they have short intestine. The alimentary canal begins at the mouth
(buccal or oral cavity), passes through the pharynx, oesophagus or food pipe, stomach, small
intestines, large intestines, rectum and finally ending at the cloaca. The food particles get digested
gradually as they travel through various compartments of the alimentary canal.
Respiratory system
The amphibian has two modes of respiration – cutaneous respiration and pulmonary respiration.
In an aquatic ecosystem, the skin is the respiratory organs where the diffusion of dissolved oxygen
takes place. This is called cutaneous respiration. While on land, they use both skin and lungs for
respiration. During pulmonary respiration, air entering through nostrils passes to the lungs via the
buccal cavity. But during summer and winter sleep, they use only skin for respiration.
Circulatory system
Frogs have a well-developed muscular heart with three chambers- two atria and one ventricle.
Blood and lymph help in the transportation of food, air and other substances throughout the body
via the network of blood vessels. The blood is composed of plasma and blood cells (RBC, WBC,
and platelets).
Excretory system
The frog is a ureotelic animal whose major excretory product is urea. They have a distinguishable
excretory system composed of a pair of kidneys, ureters, cloaca and urinary bladder. The kidneys
have the structural unit called nephron which filters the blood and excretes out the waste.
Coordination system
The nervous system and the endocrine system together perform the control and coordination in
frogs. The endocrine system is composed of the endocrine glands such as pituitary, thyroid,
parathyroid, thymus, pineal body, pancreatic islets, adrenals, and gonads. The secretions of these
glands called hormones are responsible for metamorphism and other regulatory functions.
The nervous system is divided into CNS and PNS. The brain is distinguished as forebrain,
midbrain, and hindbrain which control different parts of the body. The brain is enclosed in the
cranium and the vertebral column protects the spinal cord.
Reproductive system
Both male and female frogs have their own reproductive system where gametes for reproduction
are produced. Male frog has testes which produce sperms and eject it through the cloaca. In a
female frog, a pair of ovaries produce ovum and pass it to oviduct which opens into the cloaca.
The cloaca is a common pathway for excretion and reproduction. At a time, 2500 to 3000 eggs are
laid which are fertilized externally.
Parental Care in Amphibia
In amphibians there are many devices for the protection of the eggs during the early stages of
development and the youngs. In this way nature has practised economy in the number of eggs,
which varies in direct proportion to the chances of destruction. Parental care is the care of the eggs
or the youngs until they become able to protect themselves from the predators.
These devices fall under two heads:
(1) Protection by the parents by means of nests, nurseries, or shelters and
(2) Direct caring or nursing by parents.
The different modes of protection are given below in the three important orders of class Amphibia.
1. Protection by Means of Nests, Nurseries and Shelters:
A number of different species of frogs and toads construct nests or shelters of leaves or other
materials in which the eggs are deposited and the youngs are developed.
A. In Enclosures in the Water (Mud Nests):
A large tree frog (Hyla faber) known in Brazil as the “Ferreiro”. It protects its progeny by building
a basin-shaped nest or nursery in shallow water on the border of the pond. The female scoops mud
to a depth of 7.5 or 10 cm and with the mud, thus, removed a circular wall is built around the nest,
which emerges above the surface of the water.
The inside wall is smoothened by the flattened webbed hands and the bottom is also levelled by
belly and hands. The eggs and early larvae are, thus, protected from predators (insects and fishes,
etc.) until they are able to defend themselves. Heavy rains later on destroy the wall and larvae go
to the water.
B. In Holes Near Water (Foam Nests):
A still better mode of protecting the offspring during the early stages of development has been
adopted by a Japanese tree frog Rhacophonis schlegelii. The male and female in embrace bury
themselves in the damp earth on the edge of ditch or flooded rice field, and make a hole or chamber,
a few centimetres above water level. The walls of this chamber are polished and during this process
the gallery by which they enter into that chamber gets obliterated and then oviposition begins.
The female first produces a secretion from cloaca which is beaten into a froth. The eggs are
deposited into the froth. Now the inactive male impregnates them, and then both of them separate
and make an exit gallery towards the ditch. It is obliquely downwards towards the water, later on
this is used by the larvae who come to the water to complete the development.
The bubbles collapse, the froth liquefies and this liquid acts as an efficient vehicle for transporting
the larvae down the tunnel into the water. Similarly female of South American tree frog,
Leptodactylus mystacinus stirs up a frothy mass of mucus which is filled up in holes near water
and then eggs are laid in it. The tadpoles from these nests easily enter water. Some anuran females
discharge huge mucus and beat it into a foam with their hindlegs and then eggs are laid. Later on
hatching tadpoles drop into water from the foam.
C. In Nests on Trees (Tree Nests):
Some tree frogs like Phyllomedusa in South America, Rhacophorus malabaricus in India, and
Chiromantis in tropical Africa glues the eggs to foliage hanging over water, and after hatching, the
tadpoles drop straight into the water. Hyla resinfictrix (tree frog) lines a shallow cavity of the tree
by bees wax brought from the hives of stingless bees. Eggs are laid there when it is filled with rain
water. Tadpoles develop here safely.
Autodax (Urodela) lay 10-20 eggs in a dry hole in ground or in a hole on a tree, up to 10 metres
above the ground. Both parents remain in the hole to protect the eggs and larvae and also provide
them moisture. Youngs remain within the hole for a considerable period with their parents.
D. In Transparent Gelatinuous Bags:
The eggs of Phtynixalus biroi are large which are enclosed in sausage-shaped transparent common
membranous bag secreted by the female and is left in the mountain streams. The whole
development takes place withinA the eggs and little frogs go out in perfect condition. No gills have
been observed and the large tail serves as a breathing organ of young ones. Salamandrella
keyserlingi (urodele) deposits its small eggs in a gelatinous bag which is attached to an aquatic
plant below the water level.
E. On Trees or in Moss away from Water:
Several species of tropical American genus Hylodes lay their large eggs in damp places under
stones or moss or plant leaves. The metamorphosis is hurried up within the egg. Due to plenty of
yolk in the egg the entire development takes place within the egg and young frogs hop out as an
air breather with a vestige of tail.
2. Direct Nursing by the Parent:
A. Tadpoles Transported from One Place to Another:
Small South American frogs Phyllobates and Dendrobates and tropical African frogs Arthroleptis
and Pelobates lay their eggs on ground. The hatched tadpoles adhere by their sucker-like lips and
flattened abdomen to the back of one of their parents and, thus, they are carried from one place to
the other and in this way they can even go from one pool to the other and this is particularly when
one pond is to dry up.
B. Eggs Protected by Male:
The eggs (17 in number) of Mantophryne robusta are strung together by an elastic gelatinous
envelope forming a clump over which the male sits for development. It may be outside water .The
larvae have no gills, well developed legs, large tail which is vascular and respiratory.
C. Eggs Carried by the Parents:
In Obstetric toad (Alytes obstetricans) of Europe, the male winds the strings of eggs-formed by
adhesion of their gelatinuous investment-round his body and hindlegs. Here they are retained until
the tadpoles are ready to be hatched.
Female Rhacophorus reticulatus (Sri Lankan tree frog) carries the eggs glued to her belly.
In Desmognathus fusca (urodele) the eggs are laid in the form of rosary-like strings. The string is
bound round the body and the female nourishes them at a comparatively dry spot.
D. Eggs in Back Pouches:
(i) Exposed:
In a Brazilian tree-frog, Hyla goeldii, the female carries the eggs on the back within an incipient
brood pouch in which the eggs remain exposed. How they reached there is not known but probably
male does it. In Nototrema also the eggs are placed over the back in a single large brood pouch
covered by the skin and opened posteriorly in front of cloacal aperture.
(ii) In Cell-Like Pouches:
In Pipa americana (Surinam toad) the eggs are carried on the back of the mother. In breeding
season the back skin of female becomes thick, vascular, soft and gelatinous. The male places and
spaces the eggs. Each egg sinks into a small pouch, over which develops an operculum, which
comes from a remnant of the egg envelope, reinforced by integumental secretions.
Thus, the young develop moist and safe in maternal tissue. Between the invaginated pits arises a
rich vascularisation. In each larva there develops a broad and vascular tail. It is suspected that
metabolic exchanges take place between maternal and embryonic tissues in the manner of a
primitive placenta. The larva does not develop gills, and has been reported to be born as a tadpole
about eighty days after egg-deposition.
E. In the Mouth or Gular Pouch:
(i) By the Male:
In Rhinoderma darwini, small South American frog, the eggs (few and large) are transferred by
the male to the relatively immense vocal sacs that extend over its ventral surface. There the eggs
develop. In Arthroleptis, male frog keeps the larvae in his mouth.
(ii) By the Female:
The female of a West African tree-frog, Hylambates breviceps, carries the eggs in her mouth.
Female Rheobatrachus silus (Australian frog) keeps her eggs in her stomach. The tadpoles are
expelled through mouth after metamorphosis.
F. Coiling Around Eggs:
In Plethodon (urodele) the eggs are laid in small packages of about five beneath the stones or in
the hollow of rotten log, and the mother coils round them. In Megalobatrachus maximus (urodele)
the male coils round the eggs.
Female Amphiuma (urodele) also coils round the eggs laid in burrows in damp soil.
Coecilians Ichthyophis and Hypogeophis are oviparous, lay eggs in burrows in damp soil and coil
round them until they hatch.
G. Viviparous or Viviparity:
Two small East African toads, Pseudophryne vivipara and Nectophryne tornieri, are known to be
viviparous, but no observations have yet been made on them beyond the fact that larvae are found
in the uteri. Caecilians like Typhlonectes, Geotrypetes, Schistometopum, Chthonerpeton,
Gymnopis are ovoviviparous.
Gymnophiona
Gymnophiona, also called Apoda, one of the three major extant orders of the class Amphibia. Its
members are known as caecilians, a name derived from the Latin word caecus, meaning
“sightless” or “blind.” The majority of this group of limbless, wormlike amphibians live
underground in humid tropical regions throughout the world. Because of their relatively hidden
existence, caecilians are unfamiliar to the layperson and are not usually considered in discussions
about amphibians. They are nevertheless a fascinating group of highly specialized amphibians
about which there is still much to be learned.
Caecilians have long, limbless, cylindrical bodies that abruptly end behind the cloaca or short
tail. Annuli (primary grooves) in the skin encircle the body and form segments; in some taxonomic
groups, secondary and tertiary grooves partially circumscribe the body. Within the tissue of the
annuli, bony scales of dermal origin usually occur. The heads of caecilians are blunt, and their
skulls are bony and compact. Centres of ossification have fused, which has reduced the number of
independent cranial bones in caecilians in comparison with anurans and salamanders; for example,
a single bone, the os basale, forms both the floor of the braincase and the posterior part of the skull.
Teeth are found on all jaw bones, and a palatal series of teeth appears in addition, medial to the
maxillary series. A U-shaped facet, which articulates with the quadrate and also has a long
retroarticular process that serves as an attachment site for three major jaw muscles, is located on
the lower jaw. The vertebral column is made up of an atlas (the first vertebra of the neck) and 95
to 285 trunk vertebrae; no differentiated sacral vertebrae are present. Double-headed ribs are found
on all vertebrae except the atlas and the terminal three to six vertebrae. Of the three amphibian
orders, only caecilians have an axial musculature in which all the hypaxial components, excluding
the subvertebral musculature, form an outer muscular sheath. This sheath, which is anchored to
the skin by fibrous connective tissue, is all but disconnected from the vertebral musculature and
thereby allows the skin and superficial muscles to move as a single unit. The degenerate eyes are
covered with bone or skin. These adaptations make it possible for the caecilian to feed, reproduce,
and avoid enemies within their subterranean realm. The features of aquatic caecilians of the family
Typhlonectidae are representative of secondary adaptations.
Caecilians are aquatic and burrowing animals that superficially resemble large earthworms. Adults
range from approximately 10 to 150 cm in length. They have elongate bodies with distinct annuli,
which are grooves delineating their body segments. They are limbless, and their tails are reduced
or absent. Their eyes are reduced and are covered by skin. They are unique among the
Lissamphibia in possessing dermal scales, which occur in the annuli of some species. Their skulls
are heavily ossified and completely roofed. Caecilians possess a unique chemosensory organ, the
tentacle, which extends a short distance from the surface of the head, emerging from a skull
opening between the eyes and the nostrils.
Questions:
3. Give a note on Apoda
4. Write a brief account on respiratory mechanism of frog
10 marks
2. Describe the life cycle of frog
UNIT-II

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