THE PHYLUM ECHINODERMATA 



Fig. 16.2. External features of a starfish, 

 Aslerias Jorhesi. A, general view of the sur- 

 face of the body, showing the prominent 

 spines, the finger-like papulae or dermal 

 branchiae, and the rosettes of small white 

 pedicellariae surrounding the spines. B, 

 small region of the body, highly magnified, 

 showing these features in greater detail. 

 Note the jaws of the pedicellariae and the 

 distension of the papulae with circulating 

 coelomic fluid. (Photographs by Bassett 

 Maguire, Jr.) 



In this chapter we shall review the structure and activities of various 

 echinoderms from general and comparative points of view. 



The Class Asteroidea 



The Starfish: Structure and Activities. The commonest starfishes of the 

 Atlantic coast of North America are species of the genus Astenas; A. Jorbesi 

 occurs in shallow waters south of Cape Cod and is replaced in deeper waters 

 and north of the Cape by A. vulgaris. In general, these animals are very 

 similar. Astenas forbesi is found upon rocky and shelly bottoms, from high- 

 tide mark to a depth of about 125 feet; in these zones the bivalve mollusks 

 constituting the greater part of its food are most abundant. These starfishes 

 sometimes occur on sandy or muddy bottoms and may crawl up wharf piles in 

 search of food; on rocky shores they are often found in pools left by the 

 receding tide. 



The body consists of a central disk and five radiating arms or rays (Fig. 

 16.2). The upper or aboral surface is covered with short spines, which are 

 skeletal structures; the surface between the spines bears thin-walled, finger- 

 like projections called papulae or dermal branchiae, functioning in respiratory 



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