CLASS OPHIUROIDEA (ophis, snake + ourn, tail + andes, in the form of)
The class Ophiuroidea includes the basket stars and the brittle stars. It has two thousand species. Thus it is the most diverse group of echinoderms. But they have small body size. They live in crevices in rocks and coral. Or they are attached to algae. That is why they are not taken as an important group.
1. Arms: The arms of ophiuroids are long. They are sharply started from the central disk. It gives the central disk a pentagonal shape. Brittle stars have unbranched arms. It has a central disk that ranges in size from I to 3 cm. The arms of the basket stars are branched repeatedly.
2. The dermal branchiae and pedicellariae are absent in ophiuroids.
3. Tube feet: The tube feet of ophiuroids lack suction disks and ampullae. Some muscles are associated with the base of a tube foot. The contraction of these muscles extends the tube foot. The madreporite of ophiuroids is on the oral surface.
4. Water-vascular sy stem: Water vascular system of ophiuroids is not used for locomotion. Rather their skeleton is modified and is used for grasping and movement.
5. Superficial ossicles originate on the aboral surface. They cover the lateral and oral surfaces of’ each arm. The ambulacral groove is closed. This closed groove contains radial nen e, hemal strand and radial canal.
6. Amhulacral ossicles in the arm form a central supportive axis. Successive ambulacral ossicles articulate with one another. I.arge muscles act on them and produce snake like movements. Thus the arms curl around a stalk of algae or limn hook into a coral crevice.The central disk is kept above the substrate during locomotion. Two arms pull the animal forward. Other arms extend forward or trail behind the animal.
1. lngestion: Ophiuroids are predators and scavengers. They use their arms and tube feet in sweeping motions. They collect prey and decay matter during this movement. They then transfer this food into the mouth. Some ophiuroids are filter feeders. They wave their arms and trap plankton on mucus covered tube feet. Trapped plankton is passed from tube foot to tube foot along the length of an arm. ‘They finally reach the mouth.
2. Digestion: The mouth of ophiuroids is in the center of the central disk. It has five triangular jaws. These jaws form chewing apparatus. The mouth opens in to a sac like stomach. Intestine is absent in them. No part of the digestive tract extends inti the arms.
3. Transportation of food: Ceolom of ophiuroids is reduced. It is confined to the central disk. But it is still distribute the nutrients, wastes, and gases. Coelomocytes help in the distribution of nutrients and the removal of wastes.
Ammonia is the primary nitrogenous waste product. Ammonia is removed by diffusion across tube feet and bursae. The bursae are membranous sacs. They invaginate to from the oral surface of the central disk. A slits is present in the oral disk near the base of each arm. It allows cilia to move water into and out of the bursae.
REGENERATION, REPRODUCTION, AND DEVELOPMENT
Regeneration and Asexual reproduction
Ophiuroids can regenerate lost arms. If a brittle star is grasped by an arm, the contraction of certain muscles separate the arm form the body. Hence they are named as brittle star. This process is called autotomy (Or autos, self + winos to cut). Brittle star use autotomy to escape from prey. It later regenerates the arm. Some Species have a fission line across their central disk.Ophiuroids split into halves along this line and two ophiuroids are formed by regeneration.
Sexual reproduction and development
Ophiuroids are dioecious. Males are smaller than females. Females carry the males. The gonads are attached with each bursa. The gametes are released into the bursa. Eggs may be shed to the outside. Or they may be retained in the bursa. They are fertilized and stay there during early development. Embryos are protected in the bursa. They are sometimes nourished by the parents. Embryo change into a larva called an ophiopluteus. It is panktonic (floating). Its long arms have ciliary bands. These cilia are used to feed on plankton. This larva undergoes metamorphosis and sinks into substrate.