124 ECHINODERMA. ECHINOIDEA 



ambulacra near the peristome are depressed and leaf-like, 

 whilst the intervening interambulacra are convex ; this 

 part of the corona has consequently a petaloid appearance, 

 and is known as the floscelle. 



At the commencement of the alimentary canal there 

 is, in some genera, a complicated calcareous apparatus 

 which functions in mastication ; this is composed chiefly of 

 five jaws, which are moved by means of muscles attached 

 to ridges at the margin of the peristome — these ridges 

 constitute what is known as the perignathic girdle. This 

 may consist of ridges arising from the interambulacral 

 plates only, or there may be also processes from the sides 

 of the ambulacral plates, and these often unite, forming an 

 arch or auricle over each ambulacral area at the margin of 

 the peristome. The first part of the alimentary canal 

 passes through the centre of the masticatory apparatus. 

 The circular vessel of the water-vascular system forms 

 a ring round the oesophagus at the top of the masticatory 

 apparatus; this ring gives off five radial branches which 

 pass through the auricles and up the inside of each 

 ambulacral area. 



In the irregular echinoids there is a well-marked 

 bilateral symmetry ; a plane which passes through the 

 anus (which is in the posterior interambulacral area), the 

 apical disc, and the mouth, divides the body into two 

 similar parts. The ambulacra in these forms often differ 

 considerably in size, and, to some extent in structure ; the 

 anterior one may be smaller than the others and frequently 

 has a different arrangement of pores, while the four other 

 ambulacra are paired. The interambulacra are also un- 

 like — the posterior one often forming a large part of the 

 base of the test. The bilateral character is not conspicuous 



