346 



ECH1NODERMATA 



[CH. 



to) is covered with leathery skin and has the small anus in the 

 centre of it and is called the periproct (Fig. 167) : it is this area 

 which corresponds to the whole upper surface of the star-fish. The 

 other opening is in the centre of the lower surface and is likewise 

 covered by flexible skin ; it surrounds the mouth and is called the 

 peristome (Fig. 166). 



The cuirass itself is called the corona and consists of twenty 

 strips, each made up of a row of plates. Corresponding to each 

 tube-foot area or radius there are two rows of so-called ambulacral 

 plates, and each intervening area or interradius is similarly covered 

 by two rows of large plates. As in Ophiuroids, there is no ambu- 

 lacral groove visible from the outside : it is represented by the 

 epineural canal, immediately inside which there is the radial 

 nerve-cord. It is necessary to bear this in 

 mind when the term ambulacral plate is 

 used ; the so-called ambulacral plates of an 

 Urchin do not correspond to the similarly 

 named plates in the star-fish, as they do not 

 roof in the ambulacral groove, but form a 

 floor for it. Inside the nerve-cord there is 

 a single radial perihaemal canal as in the 

 Brittle-star (2, Fig. 164 B). As the plates 

 of the skeleton are not movable on one 

 another nothing corresponding to the am- 

 bulacral muscles of the star-fish exists, at 

 least over most of the radius, and the radial 

 perihaemal canal is separated from the 

 general coelom only by a thin septum in 

 which the radial water-tube is embedded. 

 For the same reason there is no recognisable 

 coelornic nervous system. 



If the continuous cuirass of the Sea- 

 urchin and the closed ambulacral groove 

 remind one of an Ophiuroid, the resemblance 

 ends there ; for in the Urchin the ectoderm, 

 consisting of long slender cells with a tangle 

 of nerve- fibres at the base, is spread over the whole surface outside 

 the skeleton, just as in a star-fish. This sensitive layer controls, 

 it is found, the movements of the spines, which are among the most 

 important organs of the Urchin. These spines, unlike the spines of 

 the star-fish or Brittle-star, have hollow bases, which articulate with 



Fm. 161. A glandular or 

 gemmiform pedicellaria 

 from E. esculentus x 18. 

 From Chadwick. 



