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Echinoderms

The Echinoderms

Evolutionary perspective

They are flourished 400 million year-old seas. Many were attached suspension feeders. 12 or 18 classes now extinct. Remaining are major component of marine ecosystem. Today this phylum contains common animals like sea star, sea urchins, sand dollars and sea cucumbers. Living Echinoderms still have great importance.

Fig: Evolutionary relationships of echinoderms to other animals

Characteristics of phylum Echinodermata

·         One of the strangest and most unusual of all the phylums in the animal kingdom.

·         Echinoderms are deuterostomes, which they have in common with the chordates.  All of the other invertebrates we learned about this semester have been protostomes.

·         Found on the seashore, burrowed in sand or deep sea sediments or under rocks and kelp

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·         These are spiny skin animals.

·         These are exclusively marine Animals.

·         These have triploblastic coelom.

·         The Echinoderms have bilateral symmetry in the embryonic condition, but in adult condition they are radially symmetrical(penta-radial).

·         The echinoderms have water-vascular system composed of water filled canals.

·         No cephalization or brain or central nervous system, very few specialized sensory organs. The nervous system consists of nerve net, nerve ring and radial nerves.

·         The echinoderms have Hamal system derived from coelomic cavities.

·         They have complete digestive system. But this maybe secondarily reduced.

·         The exoskeleton is calcareous (ossicles). These ossicles are derived from endodermal tissues.

·         Development begins with a free-swimming, bilateral larva, and a metamorphosis into an adult with radial symmetry.

Echinoderm Characteristics

Echinoderms have 6500 species. 13,000 are fossils. Classes are Class Crinoidea, Class Asteroidea, Class Ophiuroidea, Class Concentricycloidea, Class Echinoidea and Class holothuroidea.

Echinoderms have following characteristics.

1.       Symmetry

2.       Exoskeleton

3.       Water-vascular system

4.       Hamal system

Symmetry

In adult condition they have radial symmetry. The radial symmetry that they have is called pentaradial symmetry. Their body parts are arranged in fives or multiple of fives around oral-aboral axis. Some are secondarily bilateral symmetry. Radial symmetry allows a uniform distribution of sensory, feeding and other structures around the animal.

Fig: Arrangement of the body in fives means skeletal joints are not directly opposite one another.  This arrangement may make the skeleton stronger than if joints were opposite one another.

 Exoskeleton:

The echinoderm skeleton consists of a series of calcium carbonate plates called ossicles. These plates are derived from mesoderm, held in place by connective tissues, and covered by an epidermal layer. If the epidermal layer is abraded away, the skeleton may be exposed in some body regions. The skeleton is frequently modified into fixed or articulated spines that project from the body surface.

The evolution of the skeleton may be responsible for the pentaradial body form of echinoderms. The joints between two skeletal plates represent a weak point in the skeleton (in the following figure). By not having weak joints directly opposite one another, the skeleton is made stronger than if the joints were arranged opposite each other.

Water vascular system

ü  Echinoderms have a unique system of canals and specialized tube feet that make up water vascular system.

ü  The water vascular system’s primary function is for locomotion and for gathering food.

ü  Additionally the water vascular system also plays a role in respiration and excretion.

ü  Water vascular system originates in embryo  as a modification of the coelom. It ciliated internally.

ü  It also play role in attachment.

ü  And also play role in sensory functions.

Water vascular system has following parts.

ring canal:

The water vascular system has a ring canal. This ring canal surround the mouth.

Stone canal:

The ring canal opens outside the body through stone canal.

Madreporite:

The ring canal usually opens to the outside or to the body cavity through a stone canal and a sieve like plate, called the madreporite. The madreporite may serve as an inlet to replace water lost from the water-vascular system and may help equalize pressure differences between the water-vascular system and the outside.

tidemann-bodies:

Tiedemann bodies are swellings often associated with the ring canal. They are believed to be sites for the production of phagocytic cells, called coelomocytes. They contain phagocytes; remove foreign matters such as bacteria from incoming water.

polian vesicles:

These are sac like structures. They are also associated with the ring canal. Their function is storage of fluid for the water vascular system.

Radial Canals:

Five 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 the an arch towards the aboral pole.

Lateral canals:

Lateral canals arises from the radial canal. They end at tube feet.

Tube feet: (also called podia)

Tube feet are the extentions of canal system. They emerge through  openings in skeletal ossicles.

The tube feet end in bulb like muscular ampulla. The ampulla contracts and it forces water in tubefeet. Thus the tube feet extends. Valves prevent the backflow of water from the tube feet in to the lateral canal.

The tube feet has suction cup at its distal end  . the muscles of suction cup contract and creat a vacuum. Thus foot extend and attach with substrate. In some, extend their tube feet into a soft substrate. It forms a string contact during locomotion. It is also used to sift sediment during feeding.

Fig: the water vascular system

Fig: tubefeet