Give a brief account of hemichordates.

Explanation

Hemichordata was earlier considered a sub-phylum under phylum Chordata. But now it is placed as a separate phylum under the non-chordates. .This phylum consists of a small group of worm-like marine animals. They are bilaterally symmetrical, triploblastic and coelomate (enterocoelous) animals. The body is cylindrical and is composed of an anterior proboscis, a collar and a long trunk. A median buccal diverticulum, the stomochord (once believed to be the notochord) extends into the proboscis. Circulatory system is of open type with a dorsal heart. Respiration takes place through paired gill slits opening into the pharynx: Proboscis gland acts as the gill slits opening into the pharynx. Proboscis. gland acts as the excretory organ. Development is indirect and includes a tornaria larva.