Phylum Hemichordata are small soft-bodied creatures, living singly or in group on sandy and muddy sea bottoms or in open water. The body and coelom are divided into three regions with paired gill-slits and nervous tissue in both dorsal and ventral epidermis. Certain tissue as an anterior projection from mouth cavity was formerly interpreted as a notochord and thus placing the group with phylum Chordata. Recent studies cast doubt on such interpretation and so-called earlier notochord is termed the buccal pouch. Now this group is placed under independent phylum Hemichordata. Between hemichordates and chordates there are resemblances in the pharyngeal gill-slits and the collar cord of the nervous system- its origin form dorsal epidermis. The embryo and early larvae of hemichordates and asteroid echinoderms are much alike in ciliated bands, digestive tracts, form, derivation of anus from blastopore (deuterostome) and enterocoelous coelom. The above features strongly suggest a common origin.
DIAGNOSTIC CHARACTERS OF PHYLUM HEMICHORDATA
- Small soft bodied, marine enterocoelus coelomate creatures.
- Notochord confined to head region.
- The anteroposterior axis is divided into three parts: the anterior prosome, the intermediate mesosome, and the posterior metasome.
- The mouth is located between the proboscis and the collar.
- The trunk is the longest part of the animal.
- The prosome of pterobranchs is specialized into a muscular and ciliated cephalic shield used in locomotion and in secreting the coenecium.
- They have a diverticulum of the foregut called a stomochord
CLASSIFICATION OF PHYLUM HEMICHORDATA
- Book of Practical Zoology VERTEBRATE by S.S.LAL – Rastogi Publications.