188 



SEA-URCHINS. 



similar to those we have referred to as existing in the 

 urchins. Both these points of resemblance are, indeed, 

 indicative of an intimate relationship between urchins and 

 star-fish. And, as a matter of fact, the ambulacral areas 

 of the one represent the five rays of the other ; the inter- 

 mediate areas of the urchins being an addition to the 

 structure of the star-fish. At first sight it looks, indeed, 

 as if these animals were really symmetrically radiate, but 

 we shall show later on that this is not the case, and that 

 they are, in truth, bilaterally symmetrical like the higher 

 animals, although this bilateral symmetry has been more 

 or less thoroughly masked by the radiate arrangement of 

 the parts. 



Urchins and star-fishes are, however, not the only 

 members of the group of Echinoderms ; since this also 

 comprises the beautiful stone-lilies or crinoids,* the joints 

 of the stems of which form the well-known " St. Cuthbert's 

 beads," of the Whitby lias, while the so-called entrochal 

 marble, so often employed for chimney-pieces and other 

 decorative architecture, is almost entirely composed of 

 these stems. There are, moreover, certain entirely extinct 

 types, such as the Cystoids and Blastoids, of which more 

 anon. 



Before proceeding to trace the modifications which the 

 test of the urchins undergoes in different members of the 

 group, it may be observed that in all urchins, whether 

 recent or fossil, the number of meridional areas is in- 

 variably ten ; while in every existing kind of urchin, no 

 matter what be its size or shape, the number of rows of 

 plates composing such areas never departs from twenty. 

 As soon, however, as we reach the strata lying below the 

 chalk and gault, known as the lower greensand, which 

 constitute the lower part of the Cretaceous system, we find 

 an urchin which departs somewhat in the last-named 

 respect from the existing type. A side view of the test of 

 this species is given in Fig. 58, from which it will be seen 

 that while at the apical pole the number of meridional 

 rows of plates is the normal twenty, as we approach the 



* See page 157. 



