ECHINODERMATA. 205 



FIG. 95. 



FIG. 95. Starfish, from chart of Leuckart and Nitsche. 



Questions on the figure. How would you describe the symmetry 

 of the animal? Identify and name, by comparison with the diagrams and 

 the text, all the structures which show in the figure. Compare this with 

 specimens or figures of the common American species and note the chief 

 differences. 



symmetrical halves. No other plane does this. The two arms 

 embracing the madreporic body are known as the bivium, the 

 remaining three, the trivium. In some of the echinoids the 

 bilateral symmetry becomes much more pronounced than in 

 star-fish. 



238. The integument consists of an outer ectodermal por- 

 tion which is often ciliated (cilia wanting in the holothuroids 

 and ophiuroids), and a subepithelial, mesodermic layer in 

 which is developed the calcareous ossicles. These may occur 

 as spicules, as rods, or as plates in the various classes. Fre- 

 quently the ossicles bear spines which may or may not be 



