Lesson 7 of 12 · 7 min
Echinoderms and hemichordates
NCERT §4.2.9, §4.2.10
In a tide pool Ammu finds a five-armed star fish clinging to a rock. When she turns it over, hundreds of tiny soft tubes wave along the grooves on its underside. Her teacher also shows her a photograph of a worm-like animal that lives in sandy burrows by the sea.
The lesson in notes
In short
Echinodermata (spiny-bodied) are named for their endoskeleton of calcareous ossicles. All are marine, organ-system level, triploblastic and coelomate.
Adult echinoderms are radially symmetrical but their larvae are bilaterally symmetrical.
The gut is complete, with the mouth on the lower (ventral) side and the anus on the upper (dorsal) side.
Their most distinctive feature is the water vascular system, used for locomotion, capturing and transporting food, and respiration. There is no excretory system.
Sexes are separate and reproduction is sexual. Fertilisation is usually external, and development is indirect with a free-swimming larva. Examples: the star fish Asterias, the brittle star Ophiura, the sea urchin Echinus, the sea cucumber Cucumaria and the sea lily Antedon.
Hemichordata was once a subphylum of Chordata but is now a separate phylum among the non-chordates. In the collar region it has a rudimentary stomochord, a structure similar to a notochord.
Hemichordates are few in number: marine, worm-shaped animals that are bilateral, triploblastic, coelomate and at the organ-system level. The cylindrical body has three parts: proboscis, collar and a long trunk.
Circulation is open, respiration is by gills and the proboscis gland is the excretory organ. Sexes are separate, fertilisation external and development indirect. Examples: Balanoglossus, Saccoglossus.