168 MANUAL OF ZOOLOGY sect. 



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 Asterias, and 

 in the latter case (Figs. 91 and 92) the central disc is cor- 

 respondingly increased in extent. In some extreme instances 

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



Fig. 92. — Anthenea. Viewof ventral surface. ■ From New Zealand. (After Sladen.) 



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 ; and 

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

 narrow ambulacral grooves run out from it in a radiating 

 manner to the extremities of the arms. In some starfishes 

 there are but two rows of tube-feet in each arm. An anus 



