168 



MANUAL OF ZOOLOGY 



SECT. 



respondingly increased in extent. In some extreme instances 

 of this modification the Starfish assumes the form of a five- 

 angled disc in which the arms are represented merely by the 

 angles. In all cases the arms are hollow, each containing 

 a prolongation of the body-cavity containing the caeca; 

 in all the mouth is in the centre of the ventral surface, and 



FIG. 86. Anthenea, view of ventral surface. (After Sladen). 



narrow ambulacral grooves run out from it in a radiating 

 manner to the extremities of the arms. An anus is sometimes 

 absent. The spines and pedicellarise differ in their form 

 and arrangement in different kinds of Starfish, as do also, 

 though in a less degree, the tube-feet. The Starfishes con- 

 stitute one of the five classes of living Echinodermata, the 

 class Asteroidea. 



