544 



ECHINODERMATA ECHINOIDEA 



CHAP. 



spines ; they are of the tridactyle, the gemmiform, and the ophi- 

 cephalous types, but they have only two jaws. 



The ambulacral areas on the upper surface of the test can be 

 distinguished only by the flattened respiratory tube-feet (Fig. 

 24:2, pod'), which can be seen protruding from between the spines. 

 Below these areas are clearly marked, for in the centre of each 



pod 



Fig. 242. Aboral view of the " Sand-dollar " (Echinarachnius parma), with its spines. 

 m.p, Madreporite ; pod, small tnbe-foot with sucker ; pod', flattened respiratory 

 tube-foot. x 1. 



is a well-marked groove proceeding inwards to the peristome. 

 This groove receives lateral branches on its course which traverse 

 the adjacent interambulacral regions. The purpose of these 

 grooves will be explained later. The interambulacral regions do 

 not reach the peristome, which is entirely surrounded by the 

 ambulacral areas. The ambulacral and interambulacral areas 

 both consist of somewhat large hexagonal plates, except in the 

 region of the respiratory tube-feet. Here the pore-plates are not 

 united with one another. This region in each radius is termed 





