Phylum : Hemichordata / Stomachordata

PHYLUM: HEMICHORDATA / STOMOCHORDATA

Hemichordata was earlier considered as a sub-phylum under chordata. But now it is placed as a separate phylum under non-chordata.

The phylum consists of a small group of worms like marine animals with organ system level of organisation.

Its characteristics are :

(i) In hemichordata or stomochordata, true notochord is absent. gill slits are present but they are dorso lateral in position. They are worm like. bilaterally symmetrical. triploblastic and entero-coelomate animals.

(ii) Stomochord is a hollow outgrowth arising from the roof of the buccal cavity, also called 'buccal diverticulum'. It is present in the proboscis.

(iii) Dorsal heart, ventral hollow nerve cord, no respiratory pigments. Circulatory system is open. Respiration by gills.

(iv) Development is mostly indirect through a free swimming tornaria larva. e.g. Balanoglossus (acorn, tongue worm), Glossobalanus. Sexes are separate. Fertilization is external.

(v) Believed to be connecting link between non-chordates and chordates.

(vi) Due to the absence of true notochord in hemichordata, many taxonomists do not consider these animals as chordates.

(vii) Excretory organ is proboscis gland.

(viii) The body is cylindrical and is composed of an anterior proboscis, a collar and a long trunk.

Examples : Balanoglossus, Saccoglossus