458 



ECHINODERMATA ASTEROIDEA 



anp 



of the power of climbing. Starfish which have undergone this 

 change live on sandy bottoms and run over the surface of the 

 sand. They are also incapable of forcing asunder the valves of 

 Molluscs, and hence are compelled to swallow their prey whole. 



" Polian vesicles," or stalked sac-like outgrowths of the water- 

 vascular ring, are absent from the Asteriidae, but are found in 

 many families the Asterinidae, Solasteridae, Astropectinidae, 

 for example. They project outwards from the water-vascular 

 ring in the interradii; when there are several present in one 

 interradius they often arise from a common stalk. Cuenot 



believes that their sole func- 

 tion, like that of Tiedemann's 

 bodies, is to produce amoebo- 

 cytes, but this appears unlikely. 

 It is more probable that they 

 act as store-houses of fluid for 

 the water- vascular ring. 



The stone -canal is rarely 

 repeated, but this occurs in the 

 aberrant genus Acanthaster, 

 where there may even be several 

 in one interradius, and each 

 stone-canal has an axial sinus, 

 genital stolon, and madreporite 

 annexed to it. According to 

 Cuenot, in Asterias, when 6- 

 rayed specimens occur in a 

 species normally 5 -rayed, there are two stone-canals, suggesting 

 that the repetition of stone-canals is a suppressed effort at multi- 

 plication by division. This is also true of Uchinaster, but in 

 Ojphidiaster two madreporites may occur in an individual with five 

 arms. In the Asterinidae the Y-shaped fold which projects into 

 the cavity of the stone-canal is feebly developed, whereas in the 

 Pentacerotidae it meets the opposite side of the stone-canal, and 

 in Culcita gives out branches which reduce the cavity of the 

 canal to a series of channels. In Echinasteridae and some 

 Asterinidae, and in Astropectinidae and Pentacerotidae the 

 ampullae become so deeply indented as to be almost divided 

 into two, so that each tube-foot has virtually two ampullae. 



The alimentary canal has a remarkably constant structure. 



Fig. 196. Dissection of Gtenodiscus to 

 show the Polian vesicles, amp, Ampullae 

 of the tube-feet ; nerv. circ, nerve-ring ; 

 Pol, Polian vesicle ; sept, interradial 

 septum ; stone c, stone-canal ; T, Tiede- 

 mann's body ; w.v.r, water- vascular 

 ring, x 1. 



