ECHLKOIDEA. 295 



after the manner of a shell, and carries movable S2)ines. There is 

 invariably a mouth and anus, and locomotive and often respiratory 

 ambulacral appendages. 



The dermal skeletal plates are connected together so as to form a 

 firm immovable shell, which has no arm-like prolongations in the 

 direction of the rays, and is sometimes regularly radial, sometimes 

 irregular or symmetrical. With some rare exceptions among the 

 fossil Perischcechinidce, as Lepidocentrus, the calcareous plates are 

 firmly connected with one another by sutures, and are usually 

 arranged in twenty meridional rows. These rows (fig. 206) are 

 disposed in pairs, and correspond alternately with the radii and the 

 .inter-radii. The five radial pairs are the ambulacral plates, and are 

 pierced by rows of fine pores for the exit of tube feet (fig. 212, P), 

 and bear, as do the broad interambulacral plates, spherical promi- 

 nences and tubercles to which the differently shaped spines are 

 rnovably articulated. The body form of the Sea-urchins, as con- 

 trasted with that of the Star-fish, depends upon the meridional 

 arrangement of the rows of plates, and, at the same time, on the 

 continuity of the interambulacral rows. 



The position of the nerves and ambulacral vascular trunks beneath 

 the skeleton is the special characteristic of the internal organization. 

 Pedicellarice are found between the spicules, and are especially 

 numerous in the region of the mouth. Some Cidaridea-SiYe provided 

 with branched respiratory tubes. The genital pores are disposed 

 inter-radially on the genital plates near the apical pole. One of 

 these genital plates is, as a rule, also the madreporic plate. The 

 ocular plates, which are radial in position, are also pierced (figs. 206, 

 212). The regular Sea-urchins are often symmetrical, one radium 

 being longer or shorter than the others, which are equal to each 

 other. So we find long oval forms which are laterally symmetrical, 

 having the mouth and anus central, and an anterior unpaired radius 

 (Ac?-ocladia, Ecldnometra]. In the irregular Sea-urchins the anus is 

 thrust away from the apical pole into the unpaired radius (Clypea- 

 stridce). The mouth also often has an eccentric position in front 

 (Spatangidce, fig. 208), in which case the masticatory apparatus is 

 always wanting. 



In many regular forms all the ambulacral feet have the same 

 shape, and are provided with a suctorial disc supported by calcareous 

 bodies; in others the dorsal feet are unprovided with a disc, and 

 are pointed and often have an indented edge. In addition to the 

 ambulacral feet, the irregular Sea-urchins almost all possess arnbu- 



