140 Introduction to Animal Morphology. 



anterior, or even of this and the right and left posterior 

 as well (Abatus, Palaeostoma). In Rotula, the madre- 

 poric plate is fused with all the united genital plates. 

 Exceptionally it may unite with an ocular. The oral 

 opening is larger than the apical, and the auricles 

 project into the cavity around its border to support 

 the dentary apparatus. In discoidal forms, pillars 

 and processes along the ambulacra support the surface 

 of the shell. 



The surface spines are movable, and vary in length 

 from 2 J times the diameter of the corona to a few 

 lines, and from thick clubbed forms to fine hairs. 

 They may be smooth, granular, ribbed, or verticillate. 

 When young, they are covered by cilia, and on section 

 show concentric laminae of regularly netted, calcified 

 tissue,* with meshes filled with protoplasm. The 

 proximal end of each spine has a socket for the spine- 

 wart. The largest have a round inter-articular liga- 

 ment from the summit-pit of the wart to the bottom of 

 the socket. Each spine joint is surrounded by a 

 capsular ligament from the edge of the socket to the 

 spine ring, outside which is a layer of longitudinal 

 muscular fibres [inotores aciilci) capable of moving the 

 spine. These are covered by ciliated epithelium and 

 pigment cells. These rays are rather support- than 

 locomotory-organs. 



Among the spines in Spatangidoe are linear areas covered 

 with fine, ciliated, clubbed bristles, with a calcareous axis 

 (semitse or fasciol^e). These may surround all (s. peripetalse) 



* Ilackintosh has shoAvn that the pattern of the network is generically 

 and possibly specifically characteristic. As the spines of some Diadematidse 

 produce stinging sensations, possibly the protoplasm contains cnidse 

 {Wright). 



