Echinoderms are triploblastic, coelomate animals with radial symmetry and spiny skin. They possess a water vascular system with tube feet which are used for locomotion and food capturing. Brain and excretory organs are absent. Nervous system is diffused type. Power of regeneration of lost part is present. Dermal skeleton in the form of calcareous ossicles is present.
1. Subphylum HOMALOZOA
The group existed in the Paleozoic period and is now extinct. They were bilaterally symmetrical ancestors of all extant echinoderms.
2. Subphylum ASTEROZOA
Free living echinoderms with pentaradial arms. The group includes star fishes and brittle stars.
They are commonly called star fishes or sea stars because of star-like pentamerous shape of body. Ambulacral groove is present. Locomotion is by tube feet and respiration by dermal branchiae. Pedicellariae are present. Ex. Asterias, Asterina, Astropecten.
No pedicellariae, dermal branchiae, ambulacral grooves, intestine and anus. Visceral organs are confined to the central disc. Stomach is sac-like and pouched. Podia are used for feeding and respiration and not for locomotion. Madreporite is located on the oral side. Ex. Ophiothrix, Gorgonocephalus,Ophioderma.
Commonly known as sea daisies, they have disc-shaped body and are less than 1.0 cm in size. They show pentaradial symmetry, without arms. Podia are located on the margin. Madreporite present on the aboral side. Digestive system consists of a blind sac-like stomach, without intestine or anus. Ex. Xyloplax.
3. Subphylum ECHINOZOA
Echinoderms without arms. Mouth is ventral and anus dorsal. Podia are both locomotory and respiratory. Masticatory apparatus called Aristotle’s Lantern is present.
Arms absent. Pedicellariae 3-jawed. Ventral tube feet with suckers for locomotion and dorsal tube feet for respiration. Specialized masticatory apparatus called Aristotle’s Lantern is used for scraping and masticating algae from rocks.
Autotomy absent. Pedicillariae are 3-jawed.
Body ball-like with spines and rows of ambulacral plates. Madreporite is on the oral side and anus is aboral. Aristotle’s Lantern is well developed for feeding on algae. Respiration is through dermal branchiae and podia but some species have gills. They are adapted for hard bottom.
Body circular and dorsoventrally flattened, adapted for sandy bottom. Short, club-shaped spines are used for locomotion. Podia meant only for respiration. Mouth in the middle on the ventral side but anus is eccentric on the edge. They are commonly known as sand dollars and sea biscuits. Ex. Clypeaster, Mellita, Echinocardium.
Commonly known as sea cucumbers, found on the sea bottom and feeding on detritus. Ventral ambulacra with locomotory podia, while dorsal podia are respiratory in function. Madreporite is internal. A respiratory tree opens into the rectum. Dendritic tentacles on the anterior end collect food and push it into mouth. Ex. Holothuria, Cucumaria, Euapta.
4. Subphylum CRINOZOA
Echinoderms with radial symmetry and sedentary nature. Mouth and anus on the same side and facing upward.
Sedentary or almost so, attached to substratum by aboral cirri. Body consists of small pentamerous central disc called calyx. Mouth and anus located on the same side. Arms are 5 pairs and very long, covered with pinnules. Ambulacral grooves are ciliated and having podia without suckers, which are used for food collecting. Madreporite, spines and pedicellariae are absent. They are commonly known as sea lilies and feather stars. Ex. Antedon, Diplocrinus, Heterometra, Lamprometra, Decametra, Capilaster.