EVOLUTIONARY PERSPECTIVE (Echino, spiny + derma, skin +am, to beer)
The 400-million year-old Paleozoic rocks have almost all the fossils of the animals up to echinoderms. Many fossils of echinoderms were present in this era. Many ancient echinoderms are attached to their substrate. They were filter feeder. Filter feeding is found only in one class of the modern echinoderms. Today, this phylum contains common animals like sea star, sea urchins, sand dollars. and sea cucumbers. But the number of species of the echinoderms has reduced. Fossil records indicate that about 12 of 18 classes of echinoderms have become extinct. But still living echinoderms have a great importance. Members of three classes of echinoderms have flourished. They form a major component of marine ecosystems.
Characteristics of the phylum Echinodermata
1. They have calcareous endoskeleton in the form of ossicle. This ossicle is formed from endodermal tissue.
2. The adults have pentaradial symmetry. The larvae have bilateral symmetry.
3. Water vascular system composed of water-filled canals. These canals are used in locomotion, attachment and feeding
4. Their digestive tract may be secondarily reduced.
5. Hamal system is derived from coelomic cavities.
6. Nervous system consists of a nerve net. nerve ring, and radial nerves.
RELATIONSHIPS TO OTHER ANIMALS
The echinoderms, hemichordates and chordates following common characteristics:
1. They all are deutrostomes.
2. Anus is developed form blastopore.
3. The coelom is enterocoelous. It is formed by the out pocketing of the embryonic gut.
4.They have radial and indeterminate cleavage. Therefore, they have common ancestors. Unfortunately, fossils of a common ancestry of these phyla were not discovered. Therefore, it could not be proved how deutrosomes lineage was derived from ancestral diploblastic or triploblastic stocks.
The adult echinoderms are radially symmetrical. But most zoologists believe that echinoderms evolved from bilaterally symmetrical ancestors. There are two evidences of this relationship:
1.Their larvae have bilateral symmetry.
2. The extinct echinoderms were not radially symmetrical.
There are seven thousand species of living echinoderms. They are exclusively marine. They are resent at all depths in all oceans. They have following characters.
Modern echinoderms have a radial symmetry. This radial symmetry is called pentaradial symmetry. Their body parts are arranged in fives, or multiple of five, around an oral-ahoral axis. Radial symmetry is an adaptation for sedentary or slowly moving animals. It allows a uniform distribution of sensory. feeding and other structures around the animal. Some motile echinoderms have secondarily returned to bilateral symmetry.
The skeleton of echinoderms consists of calcium carbonate plates called ossicle. These plates are derived from mesoderm. The connective tissues held them in place. The ossicle comes out form the epidermal layer. If the epidermal layer is removed, the skeleton is exposed in some body regions. The skeleton may be modified into fixed or articulated spines. These spines project from the body surface.
The evolution of the skeleton produced pentaradial body form. The joints between two skeletal plates are weak point in the skeleton. The weak joints are not directly apposite one another. Therefore, this skeleton is stronger than the joints apposite to each other.
3. Water vascular system
The water-vascular system of echinoderms is composed of series of water-tilled canals. Water vascular system originates in embryo as a modification of the coelom. It is ciliated internally. Water vascular system has following parts.
(i) Ring canal: The water-vascular system has a ring canal. This ring canal surrounds the mouth..
(ii) Stone canal: The ring canal opens outside through a stone canal.
(iii) Madreporite: A sieve like plate is present at the end of stone canal. It is called the madreporite. Madreporite is used as an inlet. It replaces water lost from the water-vascular system. Therefore, these equalizes pressure differences between the water-vascular system and the outside.
(iv) Tiedemann bodies: These are swellings. They are associated with the ring canal. They are sites for the production of phagocytic cells, called coelomocytes.
(v) Polian vesicles are sacs like structures. They are also associated with the ring canal. Their function is storage of fluid for the water-vascular system.
(vi) Radial canals: Five (or a multiple of live) radial canals arises from the ring canal. Radial canals are present in the arms of star-shaped echinoderms. In other echinoderms. they are associated with the body wall. They form an arch toward the aboral pole.
(vii) Lateral canals: Many lateral canals arise from radial canal. They end at tube feet.
(viii)Tube feet: Tube feet are extensions of the canal system. They emerge through ON ings in skeletal ossicles. The tube feet end in a bulblike muscular ampulla. The ampulla contracts and it forces water into the tube foot. Thus the tube feed extends. Valves prevent the backflow of water from the tube foot into the lateral canal. The tub foot has a suction cup at its distal end. The muscles of the suction cup contract and create a vacuum. Thus the foot extends and attach with the substrate. In some echinoderms. the tube feet have a pointed or blunt distal end. These echinoderms extend their tube feet into a soft substrate. It forms a strong contact during toe motion. It is also used to sediment during feeding.
Functions of the water vascular system
(i) The water-vascular system is chiefly used for locomotion.
(ii) The original function of water-vascular systems was feeding, not
(iii) The water-vascular system has soft membranes. It allows diffusion of respiratory and nitrogenous wastes across the body wall.
4. Hemal system
A hemal system consists of strands of tissue. These tissues encircle an echinoderm near the ring canal of the water-vascular system.These tissues run into each arm near the radial canals. The hemal system is a vestigial circulatory system. But its function is unknown. It may help in the transport of large molecules, hormones, or coelomocytes. Coelomocytes are cells that engulf and transport waste particles within the body.