ZOO202 Official Handouts

Zoo202 handout full · Effort by MUHAMMAD HABIB · Pray for me · Lesson no 1 · Protochordates

The organisms belonging to the Protochordata are generally known as the lower chordates.

Protochordates are an informal category of animals (i.e.: not a proper taxonomic group), named

Mainly for convenience to describe invertebrate animals that are closely related to

Vertebrates. However, they do form a major division of Chordata. They are also known as

Acraniata because they lack a true skull.

Protochordates are members of two deuterostome phyla that are exclusively marine. The

Hemichordata, with solitary enteropneusts and colonial pterobranchs, share a ciliated larva with

Echinoderms and appear to be closely related, but they also have many chordate-like features. The

Invertebrate chordates are composed of the exclusively solitary cephalochordates and the tunicates

With both solitary and colonial forms. The cephalochordates are all free-swimming, but the

Tunicates include both sessile and free-swimming forms

Characteristics of Protochordata (Tunicates & Amphioxus)

Although the amphioxus and the tunicates are quite different, at one time it was believed that the

Amphioxus was a descendant of the tunicate larva. Recent genome analysis has called that view

Into question, but there are still many articles which consider both genera together.

Like the remaining subphylum of the chordates, the Vertebrata, the protochordates have a hollow

Dorsal nerve cord, gill slits, and a stiff supporting rod, the notochord, the forerunner of the

Backbone. The protochordates differ chiefly from the vertebrates in not having a backbone. Recent

Protochordates are thought to have evolved from the same ancestral stock as that which gave rise

To the vertebrates.

On the basis of the notochord present, they are divided into three sub-phylaHemichordata,

Urochordata and Cephalochordata.

Lesson no 2

Sub-Phylum Hemichordata

Bateson (1885) studied the embryology of some enteropneusts and proposed the

Name Hemichordata to replace Enteropneusta.

Hemichordate, any of a group of wormlike marine invertebrates closely related to both chordates

And echinoderms and usually considered to constitute a phylum, the Hemichordata. The term

Hemichordata—from the Greek hemi, meaning “half,” and chorde, meaning “string,” thus, “half-

Chordate”—was first proposed because the buccal diverticulum, a tubular outgrowth from the

Mouth cavity forward into the proboscis, or “snout,” resembled a rudimentary notochord—the

Dorsal, or back-side, supporting axis of the more primitive vertebrates. This theory has since been

Rejected, however, because it has been determined that the diverticulum bears little resemblance in

Origin and function to the vertebrate notochord. Molecular studies indicate that hemichordates are

More closely related to echinoderms than to chordates. About 90 species have been described.

The phylum Hemichordata includes the acorn worms (class Enteropneusta) and the pterobranchs

(class Pterobranchia). Members of both classes live in or on marine sediments. The Hemichordata

Consist of three classes: Enteropneusta, Pterobranchia, and Planctosphaeroidea. Enteropneusts,

Or acorn worms (about 70 species), are solitary, wormlike, bilaterally symmetrical animals, often

Brilliantly coloured. They are known as acorn worms because of the appearance of the proboscis