GENERAL ZOOLOGY 



Fig. 16.6. Generalized, sche- 

 matic life cycle of an asteroid 

 echinoderm. The gastrula 

 {A) transforms through inter- 

 mediate stages into a bipin- 

 naria larva (fi), which 

 metamorphoses into a young 

 starfish (Q. (Adapted from 

 H. Barraclough Fell, 1948, 

 Biological Reviews, vol. 23, 

 printed by permission of the 

 Cambridge University Press.) 



is then repeated through a new stepping sequence. The integration of all 

 these activities involves the coordinating functions of the central nervous 

 system. 



The skeletal system is of mesodermal origin and lies beneath the epidermis 

 of the body wall. It is thus an endoskeleton, showing similarities to the 

 skeleton of a vertebrate, rather than a cuticular exoskeleton of the type more 

 common among invertebrate animals. The plasticity of the body, as shown 

 in righting and other activities, is accounted for by the fact that the skeleton 

 of the starfish is composed of many small plates, bound together by connective 

 tissue and muscle fibers. 



The Reproductive System, Reproduction, and Development. Starfishes 

 are dioecious. The reproductive system consists of five pairs of gonads, either 

 ovaries or testes, one pair lying in the coelomic cavity of each arm, lateral 

 to the pyloric caeca. Each gonad is continuous with a short stalk which forms 

 its duct and attaches it to the body wall. The ducts open to the outside at 

 genital pores located deep in the angles between the bases of adjacent arms. 

 Gametes are discharged into the sea, where fertilization occurs. The eggs, 

 and thus the zygotes, contain very little yolk, and cleavage is total and 

 equal. Development proceeds rapidly through blastula and gastrula stages, 

 the gastrula soon transforming into a bilateral, ciliated larva, the bipinnaria 

 (Fig. 16.6). This larva has a complete digestive tract and feeds on uni- 

 cellular algae as it swims about near the surface. Its free life may last for 

 several weeks. Finally, the larva sinks to the bottom, becomes temporarily 

 attached, and undergoes a radical metamorphosis into a tiny starfish. In 

 the course of this metamorphosis, the organization of the larva changes 

 completely. The left side of the larva becomes the oral surface of the adult; 

 the old openings of the digestive tract disappear, to be replaced by a new 



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