Notes on PHYLUM HEMICHORDATA

PHYLUM HEMICHORDATA 

⚜Hemichordata (Hemi – half; chordata- notochord) was earlier placed as a sub-phylum under Phylum-Chordata. But, now it is considered as a separate phylum under Non-chordata. These are also called half chordates. This phylum consists of a small group of worm-like animal.

⚜General Features
Some important general features of phylum—hemichordata are discussed below:-

i. Habit and Habitat They are exclusive marine and mostly live in burrows.

ii. Symmetry and Body Organisation They are bilaterally symmetrical and triploblastic. They have organ system level of organisation.

iii. Body Form They are soft-bodied, cylindrical and unsegmented body is divisible into proboscis, collar and trunk. The body cavity is true coelom.
A true notochord is absent.

iv. Digestive Systems Digestive system is complete.

v.Respiraton Respiration occurs through several pair of gill slits or through the general body surface.

vi. Circulatory System It contains a dorsal heart and is open type.

vii. Excretory system It consists of proboscis gland.

viii. Nervous system It is primitive, consists mainly of an intraepidermal nerve plexus.

ix. Sensation Sensory cells of the epidermis act as sense organs.

x. Reproduction It is mostly sexual. Sexes are separate. Development is indirect with tornaria larva.

⚜ is the connecting link between echinoderms and chordates.
e.g., Balanoglossus (acorn or tongue worm), Saccoglossus, Cephalodiscus, etc.