PHYLUM ECHINODERMATA 



167 



exterior, open on the dorsal surface (Fig 82, A, Ovd.) through 

 a number of perforations on a pair of sieve-like plates, 

 situated inter-radially close to the bases of the arms. 



Other Starfishes, while resembling Asterias in most 

 respects, differ from it in a number of less important points. 



FIG. 85. Anthenea, view of dorsal surface. (After Sladen.) 



Thus, though the number of arms is usually five, in some 

 species it is eight, in others more, and in some of those with 

 more than five arms the number is inconstant. The pro- 

 portions borne by the arms to the central disc also vary 

 greatly in different kinds, the arms being in many instances 

 relatively longer, in many relatively shorter than in Asterias, 

 and in the latter case (Figs. 85 and 86), the central disc is cor- 



