ECHINODERMATA. ECHINOIDEA. 77 



upper margin of which is sometimes smooth, sometimes 

 crenulated. The base of the boss is frequently surrounded 

 by a smooth excavated space known as the areola (a). 

 The granules are smaller than the tubercles. 



Attached to the tubercles are the spines, these are of 

 different sizes and shapes in different genera and species 

 and even in the same individual, being needle-like, rod- 

 like, or flask- shaped. At the end of the spine where it 

 articulates with the mamelon there is a rounded cavity, 

 the acetabulum (fig. 20 A, a), above this comes the head (h), 

 this is limited above by a ring or collar (c) which may be 

 smooth or crenulate and which serves for the attachment 

 of the muscles that move the spine. Beyond the collar 

 and forming the greater part of the spine is the stem or 

 body (6), which may be smooth, or ornamented with ridges 

 or rows of spiny processes. Pedicellarise also occur. 

 On the surface of some sea-urchins, there are nearly 

 smooth bands covered with very minute tubercles, bearing 

 slender modified spines, these bands are termed fascioles, 

 sometimes they form a ring beneath the anus {e.g. 

 Micraster) when -they are said to be sub-anal, at other 

 times they encircle the rosette formed by the petaloid 

 ambulacra (e.g. Hemiaster) and are said to be peri- 

 petalous. 



The mouth is placed in the centre of a membrane, 

 which is covered with plates or granules, and is known as 

 the peristome. Consequently the aperture in the corona 

 of fossil specimens does not represent the mouth but the 

 peristome. The shape of the peristome varies in different 

 forms, it may be circular, oval, pentagonal, or decagonal. 

 In some genera there is at the commencement of the 

 alimentary canal, a complicated calcareous apparatus, 

 which functions in mastication ; this is connected with 



