30 THE ORIGIN OF MAN 



around. They consist of three main parts: (1) 

 the calyx or body proper, (2) the arms, and (3) 

 the stalk. The calyx may be large or small and 

 is made up of a variable number of closely 

 adjoining plates, which are arranged in a very 

 definite manner in the different forms. From the 

 upper part of the calyx arise the arms or radii 

 which are made up of single or double columns 

 of plates and may have a regular series of small 

 armlets arising from their inner edges and known 

 as pinnulae. The ambulacra are situated along 

 the inner sides of the arms and pinnulae where 

 the microscopic food is captured and conveyed 

 to the mouth at the top of or within the calyx. 

 The anal aperture is also on the upper or ventral 

 surface of the calyx but is always more or less 

 eccentrically situated and often drawn out into 

 a long anal tube. The stalk consists of many su- 

 perimposed, disc-like, perforated pieces called 

 columnals, and is usually short. 



The blastids are small, stalked, extinct Echino- 



