ix PHYLUM ECH1NODERMATA 363 



longer recognisable. Five oral plates, which when they first appear 

 are on the abactinak^lirface^pass round to the actinal as develop- 

 ment proceeds. 



L Mr?" 



2. KXAXIPLK UK Tin: Km i \OIDF.A. 



Y 



A Sea-Urchin. (ttroiigywcentrotiis or Uehinus.) 



, 



General External Features. -The Sea-urchin (Figs. 293 and 

 294) is globular in^hape, but somewhat compressed in one direc- 

 tion, so that two poles are distinctly recognisable. At one of these 

 the degree of flattening is greater than at the other ; this is the 

 oral pole, the opposite pole being termed the anal or aloral. At 

 the oral pole is a rounded aperture, the mouth, through which may 

 be seen projecting five hard white points, the extremities of the 

 teeth. Surrounding the mouth is a thin soft membrane known as 

 the peristome or peristomial membrane. At the anal pole is a much 

 smaller aperture, the anus, the space immediately surrounding 

 which is termed the periproct. 



. The entire surface, with the exception of the peristome and 

 periproct, is bristling with spines cylindrical, pointed, solid ap- 

 pendages, the surface of which is longitudinally fluted. These are 

 movably articulated with the body so that they may be turned 

 about in all directions. When one of them is removed (see Fig. 

 309, p. 389), it is found that the joint is of the character of a ball 

 and socket, a concavity on the base of the spine fitting over a 

 hemispherical elevation of the surface of the Sea-urchin, and the 

 spine being retained in place and caused to move by means of a 

 capsule of muscular fibres enclosing the joint. Around the bases 

 of the large spines are a number of very small spinules. Here and 

 there among the spines are to be observed minute pcdiccllarice 

 (see Fig. 310, p. 389), which are comparable to the stalked 

 pedicellarise of Asterias, but have each three jaws instead 

 of two, and have a relatively long stalk, which is supported by a 

 slender calcareous rod. Here and there are to be found also small 

 rounded bodies termed the sphceridia, which are perhaps, like the 

 pedicellaria?, to be looked upon as modified spines : they contain 

 ganglion-cells and are apparently organs of special sense, having 

 perhaps the function of detecting changes in the composition of 

 the water. 



Projecting from the surface among the spines all the way from 

 the peristome to the periproct will be observed five double rows of 

 tube-feet (Fig. 293), which in a living specimen will be found to 

 be capable of great extension. These are similar to the tube- 

 feet of the Starfish, and have similar functions ; the sucker-like 

 extremity of each is supported by a perforated sieve-like plate of 

 calcareous matter. Each double row of tube-feet occupies a 

 meridional zone of the surface, termed the uiiiliidnrriil area, 



