COELENTERATA GENERAL

CHARACTERS AND

CLASSIFICATION

PRESENTED

BY

Dr. Y. SAVITHRI

LECTURER IN ZOOLOGY

GOVT. COLLEGE FOR MEN(A),

KADAPA.

HISTORY OF COELENTERATA

Aristotle Knew the stinging qualities of coelenterates and

considered hese organisms as intermediate between plants and

animals and termed them Acalephe or cnide (Gr., akalephe

=nettle; cnodos = thread).

They were included in the Zoophyta ( Gr; zoon= animal; phyton=

plant) together with various forms from sponges to ascidians.

The animal nature of coelenterates was established by Peyssonel

(1723) and Trembley (1744).

Linnaeus, Lamarck and Cuvier grouped the coelenterates under

Radiata which included the echinoderms also because of their

symmetry.

Finally, Leuckart (1847) separated the coelenterates from

echinoderms and created a separate phylum Coelenterata

(Gr., koilos = cavity; enteron – intestine).

1. COELENTERATA – HOLLOW GUT

2. CNIDARIA – NETTLE

GENERAL CHARACTERS

Hatschek (1888) splitted Leuckart’s Coelenterata

into three distinct phyla – Spongiaria (Porifera),

Cnidaria (Coelenterata) and Ctenophora.

Sea

Anemone

Hydras Jellyfish Sea Coral

CONNECTING LINKS

Proteospongia:

Protozoa and Porifera

Ctenoplana:

Coelenterata and helmenthes

Coelenterates are Metazoa or multicellular animals with tissue grade of

organisation.

These are aquatic, mostly marine except few freshwater forms like Hydra.

These are sedentary or free-swimming and solitary or colonial.

Individuals are radially or bi-radially symmetrical with a central gastro

vascular cavity communicating to the exterior by the mouth.

Diploblastic animals; body wall consists of an outer layer of cells called

ectoderm and inner layer of cells the endoderm cemented together by an

intermediate layer of non-cellular gelatinous mesogloea.

These animals exhibit the phenomenon of polymorphism with very few

exceptions; the main types of zooids in polymorphic forms are polyps and

medusa. Polyp is sessile and asexual zooid, while medusa is free-

swimming and sexual zooid.

Skeleton, either exoskeleton or endoskeleton is of common occurrence.

Acoelomate animals because they do not possess a second body cavity,

the coelom.

Short and slender tentacles encircle the mouth in one or more whorls

and helps in food capturing, ingestion, locomotion and protection.

The tentacles are provided with nematocysts; tentacles serve for food

capture, its ingestion and for defence. These are also present on body

layers, these are adhesive organs.

These are usually carnivorous; digestion is extracellular as well as

intracellular. Anus is not found.

Nervous system is primitive , consisting of diffuse nerve net.

Respiratory, circulatory and excretory systems are wanting.

Reproduction is both by asexual and sexual methods.

Asexual reproduction occurs by budding and sexual

reproduction by the formation of gametes.

A ciliated planula larva usually present in the life history.

The life history exhibits the phenomena of alternation of

generations or metagenesis in which the asexual polypoid,

sessile generation alternates with sexual medusoid, free-

swimming generations.

FEEDING

Cnidarians are carnivores that use tentacles to capture prey. The tentacles are armed with cnidocytes – unique cells that function in

defense and the capture of prey. Nematocysts contain toxins used for prey capture and defense.

FEEDING

Extracellular digestion begins in the gastrovascular cavity, but is

completed within the cells of the gastrodermis.

Some cnidarians supplement their diet with nutrients collected from

algal symbionts (zooxanthellae).

LOCOMOTION

A. Medusa: motile, free-swimming

B. Polyps: sessile

Exceptions:

1. Hydra tumbles on tentacles

2. Sea anemones glide on pedal disc

RESPONSE

Sensory cells

1. Chemoreceptors (chemicals)

2. Thigmoreceptors (touch)

3. Photoreceptors (light)

Ocelli (eyespots)

4. Statocysts (balance)

NERVE NET

Cnidarians have a diffuse nervous system.

Nerve cells forming two interconnected

nerve nets in the epidermis and

gastrodermis.

No concentrated grouping of nerve cells

forming a central nervous system.

CNS does not provide advantage for

radially symmetrical animals where

stimuli approach from all sides.

REPRODUCTION

1. Asexual

budding from polyps or medusae

2. Sexual

a. Medusae release sperm & eggs

b. Some monoecious, some dioecious

c. Larvae free-swimming

CLASSIFICATION OF COELENTERATA:

The classification followed here is given by Hyman, L.H,. (1940).

According to Hyman, Phylum Coelenterata has been divided into

three classes, viz., Hydrozoa, Scyphozoa and Anthozoa.

Sub Phylum

Hydra: (Sea Fur)

Physalia:

(Portuguese man of

war)

Velella, Porpita,

Halistemma

Aurelia,

Rhyzostoma,

Lucernaria,

Pericolpa

Periphylla

Gorgonia

Adamsia

Meandrina

Metridium

Madripora

Ex: Ex:

Ex:

BROWN HYDRA EATING

Class: Hydrozoa (Gr: Hydra=water, zoon=animal

Hydrozoa are solitary and freshwater or mostly colonial and marine, sessile and free- swimming

forms.

They exhibit tetramerous or polymerous radial symmetry.

Body wall consists of an outer ectoderm and an inner endoderm separated by non- cellular

mesogloea.

Gastrovascular cavity without stomodaeum, septa or nematocysts bearing gastric filament.

Skeleton or horny structure is horny peri- sarc in some forms, while coenosarc secretes a

skeleton of calcium carbonate forming massive stony structure or coral in other forms.

They exhibit polymorphism, There are two main types of zooids, the polyp and medusa.

Medusa is provided with true muscular velum. Many Hydrozoa exhibit alternation of

generations.

Reproductive products or sex cells are usually ectodermal in origin and discharged externally.

Cleavage is holoblastic, embryo ciliated planula.

Ex: Hydra, Obelia, Porita, physalia (Portugese an of war), Velella (Little sail) Millipora.

Fried egg jelly

CLASS: SCHYPHOZOA: GR: SKYPHOS= CUP, ZOON= ANIMAL

Scyphozoa include large jelly-fishes or true medusae are exclusively

marine. Medusae are large, bell or umbrella-shaped, without true

velum, free swimming or attached by an aboral stalk. Marginal

sense organs are tentaculocysts having endodermal statoliths.

Polypoid generation absent or represented by small polyp, the

scyphistoma which gives rise to medusae by strobilisation or

transverse fission.

Gastrovascular system without stomodaeum with gastric filaments

and may or may not be divided into four inter-radial pockets by

septa. Mesoglea is usually cellular.

Gonads are endodermal and the sex cells are discharged.

Ex: Aurelia, Cassiopea, Rhyzostoma, Lucrnaria, Periphylla.

CLASS: ANTHOZOA GR: ANTHOS=FLOWERS, ZOON = ANIMAL OR

ACTINOZOA: GR: ACTIN = RAY, ZOON = ANIMAL.

Solitary or colonial exclusively marine forms.

They are exclusively polypoid. Medusoid stage is altogether absent.

Body usually cylindrical with hexamerous, octomerous or

polymerous biradial or radio bilateral symmetry.

The oral end of the body is expanded radially into an oral disc

bearing hollow tentacles surrounding the mouth in the centre.

The stomodaeum is present, often provided with one or more ciliated

grooves the siphonoglyphs.

Gastro vascular cavity is divided into compartments by complete or

incomplete septa or mesenteries. Mesenteries bear nematocysts at

their inner free edges.

Mesogloea contains fibrous connective tissue and amoeboid cells.

Skeleton either external or internal.

Exoskeleton is formed from calcium carbonate which often form massive

corals.

Nervous system is in the form of typical nerve net without a concentrated

central nervous system.

Gonads are endodermal, develop in the mesenteries.

The ripe sexual products are discharged into coelenteron.Fertilisation is

external.

The fertilised egg develops into a planula larva, which after a short free life

settles down and develops into an adult.

Ex: Metridium(Sea anemone), Adamsia (sea anemone), Corallium(Precious coral or red coral), Gorgonia (Sea fan), Pennatula (Sea pen).

ECOLOGICAL ROLE

A. Predators and prey

B. Neurotoxins in medical research

C. Coral – jewelry, building, reefs (surfing!)

D. Coral reefs - habitat for many

-great biodiversity

- protect coastline

E. Symbiosis with other organisms