BIO 5 PLANT BIOLOGY

PHYLUM ECHINODERMATA

 

1.                  The name “echinodermata” means “spiny skinned” animals. This refers to the fact that many of these animals produce spines of calcium carbonate that protrude from the surface of the animals. These spines are part of the endoskeleton of the animals.

2.                  The Phylum Echinodermata (echinoderms) includes starfish (or sea stars), sea urchins and sand dollars, brittlestars, and sea cucumbers.

3.                  Members of the phylum share several features with other animals groups. For example, the members of the Echinodermata have organ systems, a coelom, and a complete digestive system.

4.                  The members of the phylum do, however, have some distinctive or special features. First, the animals are primarily radially symmetrical. However, the opening of the water vascular system is somewhat “off-center”. Consequently, some zoologists refer to the symmetry as biradial. Echinoderms have an endoskeleton that consists of calcium carbonate (limestone) plates, some of which are modified into spines. Finally, echinoderms have a water vascular system which, in starfish, provide the primary means of movement.

5.                  All echinoderms are marine, many live in tide pools.

6.                  The starfish was used as the primary example of the phylum. Starfish are carnivores, and they eat clams, other mollusks, and an occasional fish.

7.                  In feeding, a starfish plants itself on top of a clam and begins to pull apart the shells using its tube feet. Once the shell is partly open, the starfish inserts its cardiac stomach into the interior of the clam and proceeds to release digestive enzymes into the soft body of the mollusk. As the clam is digested within its shell, the adductor muscles holding together the two halves of the shell become digested to the point where the hinged clam shell springs open. The starfish continues to ingest the partly digested clam until all edible parts of the clam have been taken into the starfish.

8.                  The starfish moves by means of its water vascular system. This consists of a pore called the madreporite through which seawater can enter the system of tubes, a ring canal that encircles the digestive tube, radial canals that extend outward into each arm, and many hundreds of tube feet. The canal system consists of hollow tubes of calcium carbonate that are filled with seawater. This system extends tube feet, allows the “suction cup”-like tip of each tube foot to attach to the substrate, and retracts the tube feet. A nervous system consisting of a nerve ring around the digestive tube and a nerve cord that extends into each are coordinates the function of the tube feet.

9.                  The complete digestive system of the starfish includes a mouth on the oral surface. (Since these are essentially radially symmetrical animals, they have no anterior, posterior, dorsal, or ventral surfaces. Consequently, the surface with the mouth is called the oral surface, and the surface with the anus is called the aboral surface.) A short esophagus leads to the large, folded cardiac stomach and subsequently to the smaller pyloric stomach, a short intestine, and the anus. A pair of extremely large digestive glands is present in each of the starfish’s arms. The digestive glands are connected to the pyloric stomach by ducts. Since digestion is partly external (in the clam shell), huge amounts of digestive enzymes are necessary to carry out this process and the vast digestive glands supply these enzymes in large amounts. Digestion is completed in the pyloric stomach, and the food molecules are absorbed into the wall of the stomach.

10.              Food distribution is done by the fluid within the coelom. The coelom is large and extends into each arm of the starfish. The lining of the coelom has cilia. These beat to circulate the fluid of the coelom. Food molecules diffuse or are actively transported from the wall of the pyloric stomach into the coelomic fluid. The churning of the fluid distributes food molecules to all parts of the starfish.

11.              Digestive waste is eliminated through the anus. Nitrogen waste is carried by amoebocytes that inhabit the coelomic fluid. The amoebocytes take the nitrogen waste to skin gills. From the skin gills, the nitrogen waste diffuses into seawater surrounding the starfish. Carbon dioxide waste also diffuses into sea water from the skin gills. The carbon dioxide waste is carried by the coelomic fluid to the skin gills.

12.              Oxygen diffuses into the coelomic fluid from sea water at the skin gills.

13.              Surrounding the skin gills are pedicellaria, pincher-like structures that remove debris from the surface of the starfish and ensure that the sites of gas exchange are not covered or obstructed in any way.

14.              The Phylum Echinodermata includes four classes. Starfish belong to the Class Asteroidea. These animals have tapering arms. The number of arms ranges from 5-20, and all are carnivores. Class Echinoidea includes sea urchins and sand dollars. These animals lack arms and have their endoskeleton plates fused into a solid shell. Sea urchins have long spines that are used for such functions as protection. Sea urchins are herbivores. Sand dollars eat organic material found in sand. Class Ophiuroidea includes the brittle stars. These animals have long, slender arms that break off easily. The arms are used for swimming. Brittle stars are carnivores. Class Holothuroidea includes sea cucumbers. These animals are either filter feeders or eat organic matter found in ocean sediments. The endoskeleton of these animals consists of microscopic plates.