Animal Kingdom - VI
PHYLUM HEMICHORDATA
. Hemichordates are vermiform, solitary or colonial enterocoelous coelomate animals having intra-epidermal nervous system and a pre-oral gut with or without gill slits and without typical nephridia.
Due to some similarities with chordates some workers (Bateson, 1885) considered
Hemichordata as a subphylum of the phylum Chordata. Alliance with the chordates was based on the presence of gill slits and the so called 'notochord
- It is now generally agreed that the hemichordate "notochord" is neither analogous nor homologous with the chordate notochord and that except the commorn possession of pharyngeal clefts the two groups are dissimilar On grounds of its similarities with invertebrates some recent scientists,like Van der Horst (1939), Dawydoff (1948), Marcus (1958) and Hyman (1959), have given it the status of an independent invertebrate phylum of a minor phylum. chordates, a fact that is undisputed.
- Since the group comprises of only about 100 species, it is included in the category
- The name Hemichordata (Gr. hemihalf, chorde-cord) means they are half or part
General characters
Exclusively marine, solitary or colonial, mostly tubicolous.
- Body soft, fragile, vermiform, unsegmented, bilaterally symmetrical and triploblastic.
- .Body wall of a single-layered epidermis with mucus glands. No dermis. Coelom enterocoelous, usually divided into protocoel, mesocoel and metacoel corresponding to three body regions i.e, proboscis, collar and trunk.
