CLASS ECHINOIDEA (Or. echinos, spiny + oeides, in the form of)
The class Echinoidea is composed of sea urchins, sand dollars and heart urchins. Echinoidea has one thousand species. They are widely distributed in all marine environments. Sea urchins are specialized for living on hard substrates. They move into crevices and holes in rock or coral. Sand dollars and heart urchins live in sand or mud and burrow. They use tube feet to catch organic matter. Sand dollars live in dense beds. It helps them for efficient reproduction ad feeding.
Sea urchins are rounded. Their oral end is oriented toward the substrate. Their skeleton is called a test. Test consists of 10 closely fitting plates. These plates form arch between oral and aboral ends. Five rows of ambulacral plates have openings for tube feet. Ambulacial plates alternate with five interambulacral plates. Interambulacral plates have tubercles for the articulation of spines. The base of each spine is a concave socket. The muscles at its base move the spine. Spines are sharp and billow. They contain venom dangerous to swimmers.
The pedicellariae of sea urchins hake two or three jaws. The pedicellarie are connected to the body wall by a long stalk. The pedicellarie clean debris from the body. They also capture planktonic larvae. They are grooved or hollow. This groove is used to inject ito a predator like sea star.
4. Water vascular system
The water-vascular system is similar to other echinoderms. Radial canals run along the wall between the oral and the aboral poles. Tube feet possess ampullae and suction tips. The water-vascular system opens outside into aboral ossicle through many pores. Aboral ossicle acts as a madreporite.
5. Locomotion and burrow formation
Echiniods move with the help of spines and tube feet. The spines are used for pushing the body against the substrate and tube feet are used for pulling the body. Sand dollars and heart urchins use spines to dig burrow in soft substrates. Some sea urchins burrow into rock & coral. Burrows protect themselves from the waves and strong currents. They form cup-shaped depressions and deeper burrows with the help of Aristotle’s lantern.
Echinoids feed on algae, bryozoans. coral polyps, and dead animal. Oral tube feet surround the mouth. They are used to capture food. Mouth has a chewing apparatus called Aristotle’s lantern. This apparatus can be projected from the mouth. Aristotle’s lantern consists of 35 ossicles and attached muscles. It cuts food into small pieces for ingestion. The mouth cavity opens in to a pharynx, an esophagus. and a long, coiled intestine. The intestine ends at the anus towards aboral side.
Circulatory, respiratory and excretory systems
Echinoids have a large coelom. The coelomic fluids are the primary circulatory medium. Small gills are present in thin membranes around the mouth. Gills are out pockets of the body wall. These gills are lined by ciliated epithelium. Gas exchange occurs by diffusion through the epithelium and the tubefleet. Ciliary currents changes in coelomic pressure. The contraction of muscles associated with Aristotle’s Lantern move coelomic fluids into and out of gills. Excretory and nervous functions are similar to asteroids.
REPRODUCTION AND DEVELOPMENT
Echinoids are dioecious. Gonads are present in internal body wall of the interambulacral plates. They nearly fill the coelom during breeding season. One gonopore is present in each of five ossicles called genital plates. The genital plates are present at the aboral end of the echinoid. The sand dollars have only four gonads and gonophores. Gametes are shed into the water. Fertilization is external. Development starts and pluteus larva is formed. Pluteus larva spends several months in the plankton. It undergoes metamorphosis and becomes adult.