Protochordates

“The first chordates”

2 Subphyla

• 1) Urochordata – the sea squirts and other loosely related groups (invertebrates).

• 2) Cephalochordata – represented by amphioxus.

Deuterostomata - “two openings” and refers to an event during embryological development.

– During embryogenesis a blastopore develops into the anus and a second opening will form during development that will become the mouth.

Basal Deuterostomes: Echinoderms and Hemichordates

Echinodermata

Hemichordata • A group of marine invertebrates commonly called

“acorn worms”.

Urochordata • A group of filter feeding, marine organisms. • Have thin, transparent, nonliving outer tunic.

Three classes of Urochordates

– Ascidians

– Larvaceans

– Thaliaceans

Class Ascidians (sea squirts)

• Tiny (between 0.5mm to 11 mm), short living (hours to days) organisms.

• Sessile animals; remain firmly attached to substratum (e.g. rocks and shells).

retain the larval form throughout life

Class Larvaceans

– Typically occur as free-floating plankton.

– Tiny -- 8mm in length.

– Have a small body and a long tail supported by a notochord.

Class Thaliaceans

– free living or colonial organisms

– resemble adult ascidians in appearance

Cephalochordates (Amphioxus)

– also known as

the lancelets

Anatomical features of Amphioxus

– The body consists mostly of a trunk.

– Has a semi-transparent skin composed of two layers.

– Amphioxus possesses pharyngeal slits.

– The slits do not serve a respiratory role but instead help to facilitate filter feeding.

– Uses the skin for respiration.

– The nervous system of amphioxus is similar to that of vertebrates.

• Amphioxus feeds by filtering food out of the water.

• The coelom is reduced in adult amphioxus.

• Amphioxus circulation differs significantly from that of vertebrates.

• Cephalochordates lack an organized kidney.

• In mature amphioxus, the gonads will be prominent and will bulge into the atrium.

• Amphioxus resembles the larval form of the lamprey called the Ammocoete.

• Like amphioxus ammocoetes have a dorsal, hollow nerve cord and a brain.

• Ammocoete trunks also display myomeres.

• Both organisms are filter feeders.

• Ammocoetes have a vascular arrangement that is more reminiscent of vertebrates but, like amphioxus, it lacks a heart.

• Ammocoetes have a kidney (although it is primitive).