PROTOCHORDATES

Ernst Haeckel proposed the establishment of Phylum

Chordata

- Sp Urochordata, Sp Cephalochordata, Sp Vertebrata

Deuterostomata

Echinodermata (phylum)

Hemichordata (phylum)

Chordata (phylum)

Urochordata (subphylum)

Ascidiacea. Sea squirts (class)

Larvacea. Larvaceans (class)

Thaliacea. Thaliaceans (class)

Cephalochordata. Lancelet or Amphioxus (subphylum)

Craniata (subphylum = subphylum vertebrata of Haeckel)

Myxini. Hagfish

Vertebrata. Craniates w/ vertebrates

BASAL DEUTEROSTOMES (ECHINODERMS & HEMICHORDATES)

1. ECHINODERMS starfish, sea squirts, sea cucumber, sea daisies &

crinoid

unique calcium carbonate skeleton

secondary radial symmetry

2. HEMICHORDATES William Bateson added Enteropneusta to

phylum Chordata which he named Hemichordata

Dorsal nerve strand and the tendency to have a lumen in the

collar region believed by Bateson to be an expression of the

same phenomenon that induces the formation of a

dorsal neural groove and tube in chordates

Have slits that open to the exterior basic feature of

chordates

Stomochord short diverticulum of the foregut

Diverticulum may be homologous to notochord of

chordates

- Suggesting the grouping of Hemichordates with Chordates is the presence of pharyngeal slits and a blood vascular system

UROCHORDATES (or Tunicata)

- notochord is confined to the locomotor tail of the free-living larval stage

- enclosed in a beautifully transparent tunic

- filter feeders

- Ascidians undergo complete metamorphosis, resorb the tail and the notochord before becoming sexually mature

- Larvaceans retain the tail and notohord throughout life and reproduce in larval-like stage

- Thaliaceans have no larval stage, no notochord, and no tail.

1. ASCIDIANS sea squirts

solitary and colonial

larvae has a fleeting existence

notochord is present

nervous system (dorsal hollow nerve cord, several ganglia & nerves)

sensory vesicle (associated w/ brain that houses an otolith & ocellus)

Otolith for stratoreception w/c stimulates nerve endings

Ocellus pigment-protected receptor cell

Metamorphosis: 3 adhesive papillae attach the larvae to permanent

substrate

Larvae

Adult

Larval mouth(Incurrent siphon (anterior)

Atriopore

(Excurrent sipohon (posterior)

2. LARVACEANS appendicularians

solitary, free-swimming planktons found in water surface

secrete mucopolysaccharides w/c surrounds organism

has a tuniclike mass that is abandoned as it gets clogged with debris from filter feeding

3. THALIACEANS the salps

free-living individuals and colonial individuals in alternate generations

resembles adult ascidians (sea squirts)

cylindrical in shape (excurrent and incurrent apertures are on opposite sides of cylinder)

differ from ascidians in having buccal and atrial opening

filter feeders

no notochordCEPHALOCHORDATA

- Amphioxus means sharp at both ends or

- Lancelet little spear a.k.a Branchiostoma

- found near sandy beaches

- burrow into sand w/ eel like movements

- filter feeders

Locomotor Musculature and Skin

body is all trunk

exhibit metamerism from myomeres (lie under the skin)

each myomere is separated from next by a myoseptum (connective tissue partition that is the origin and insertion of muscle)

utilize cutaneous respiration

Pharyngeal Slit

open in a fluid-filled cavity the atrium

pharynx does not serve a respiratory function

oxygen-rich water rapidly depletes to points where there is a local oxygen debt

Notochord

consists of muscular discs (contrast to chordates)

used to burrow (notochord is continuous to the tip of the rostrum unlike chordate)

Nervous System and Sense Organs has a central nervous system consisting a hollow brain

dorsal nerve cord containing a central canal

both lined by ependyma (nonnervous supportive membrane)

brain and cord surrounded by leptomeninx (single connective tissue membrane)

only 2 brain subdivision (in contrast with craniates that have 3)

notochord extends anterior to the brain (in craniates it stops in the midbrain)

spinal nerve consists only of dorsal roots

have abundant chemoreceptors

have tactile receptors (elicit withdrawal)

most prominent is the light-sensitive ocelli (receptor cell and caplike melanocyte)

Food Processing

filter feeders

vestibule (chamber for collecting seawater) w/c is bounded laterally by an oral hood, caudally by a perpendicular membranous velum wheel organ covered by a sticky mucus retrieve heavier food particles that miss the mouth

Coelom

not a prominent cavity in adults

becomes laterally compressed as additional pharyngeal bars form in the elongating larvae

larval coelom is prominent and metameric because it forms from the metameric outpocketings of the dorsolateral roof of the embryonic archenteron

Circulatory System

amphioxus has no heart

blood is a colorless cerum that lacks blood cells, platelets, and other formed bodies

Elimination of Metabolic Waste

amphioxus has no compact organ known as kidney

cytopodocytes cells that collect metabolic wastes (pedicels-glomerolus, microvilli)

Gonads

mature gonads are visible throughout the body

dioecious (ovary and testes do not develop in same individual)

Amphioxus and the Craniates Contrasted

Amphioxus

Craniates

-cephalization

x

-paired sense organs

x

-notochord

/

-vertebral column

x

-pharyngeal slit

/

-dorsal hollow CNS

/

-craniate subdivisions

x

-metamerism

/

(extended to anterior tip of head)

-two layered skin

/

-arterial & venous channels/

-heart

x

-coelomate

/ (restricted in adult)

ORIGIN OF CRANIATES

Ostracoderms majority of early vertebrate fossil

posses hard bony parts

had no jaws

no paired fins

filter feeders

armored fishes because of the bony plates on the skin

Cephalochordates

have a notochord, pharyngeal slits, a dorsal hollow central nervous system w/ brain and cord, metameric bodywall musculature, two-layered skin, and arterial & venous channels

are deuterostomes coelomate

filter feeders

THE AMMOCOETE: A VERTEBRATE LARVAE

have a notochord the commences in the midbrain and continous to the end of the body

the dorsal hollow CNS develop the same manner as that of anamphioxus

several gills slits open to exterior

body musculature is disposed as overlapping myomeres that provide locomotion

filter feeders

CS is similar to that of an amphioxus but ammocoete has a heart

exhibit craniate features