CHORDATA: INTRODUCTION

· Balfour introduced the term Chordata (L. chorda-cord/rod, ata-group).

Diagnostic Characteristics of Chordata

· Notochord: It is a solid, rod-like axial skeletal structure present below the nerve cord and above the alimentary canal.
It is replaced by a vertebral column in adult vertebrates.
· Dorsal tubular nerve cord: It disappears in adult forms of urochordates. It remains throughout life in other chordates.
· Pharyngeal gill slits: Perforations in lateral walls of the pharynx.
Persist throughout life in a protochordate, cyclostomes, and fishes and help in respiration.
Pharyngeal gill slits: Not found in adult amphibians, reptiles, birds, and mammals.

General characteristics of Chordata

· True coelomates. Coelom is schizocoelic in origin.
· Triploblastic, bilaterally symmetrical, metameric segmentation.
· Post-anal tail
· Paired appendages.
· Heart: muscular, ventral.
· Respiratory pigment: Hemoglobin is present in RBCs.
· Closed type of circulatory system with the hepatic portal system.

Sub-phylum: Hemichordata

· True notochord is absent, but has a primitive structure called stomochord is present which is the outgrowth of gut
· They have dorsoventrally placed pharyngeal gill slits.
· Dorsal tubular nerve cord is present in the anterior end or collar region only.
· Body is divisible into proboscis, collar, and trunk
· Indirect development has a free-swimming larval stage called Tornaria larva
· That resembles bipinnaria larva of echinoderms.

Examples: Balanoglossus (Tongue worm) and Rhabdopleura.

· It represents the connecting link between echinoderm and chordates.

Sub-phylum: Urochordata

· They are marine form. The body is covered by a test or tunic which is made up of a cellulose substance called tunicin. So-called tunicates.
· Show retrogressive metamorphosis because the larval form has only a notochord but is absent in adults.
· They are commonly known as sea squirts.
Examples: Herdmaina (Sea potato), Salpa, Doliolum

Sub-phylum: Cephalochordata

· They are marine form.
· Notochord is well-developed and extended up to the entire length of the body up to the anterior tip beyond the nerve cord so-called Cephalochordata.
· Pharyngeal gill slits open outside indirectly through an atrium and atriopore.
· They have an endostyle and ciliated groove in the floor of the pharynx used for getting food.

DIVISION: AGNATHA

· Eel-shaped body. A single class in existence i.e. Cyclostomata.
· Lack jaws (a: absent, gnathos: jaw) and paired appendages.
· Circular mouth. (Cyclos = circular, stoma = mouth)
· Two chambered hearts.
6 – 15 pouch-like gill slits (Marsipobranchii)
· Lateral line sense organs.
· Exoskeleton lacking endoskeleton cartilaginous
· Internal ear with one or two semicircular canals.
· Notochord persists throughout life.
· Ectoparasites or scavengers on fishes.
· Development through Ammocoete larva.

Examples: Petromyzon (Lamprey, a living fossil), Myxine (Hagfish), etc.

Petromyzon and Myxine

DIVISION: GNATHOSTOMATA

· Bear jaws (gnathos: jaw, stoma: mouth) and paired appendages.
· Consists of two superclasses: Pisces and Tetrapoda.