190 



SEA-URCHINS. 



and lower Silurian, we shall find a totally extinct group 

 of Echinoderms known as Cystoids. The hard parts of 

 these creatures consist of a nearly globular test, usually 

 supported on a short stalk, and composed of a number of 

 polygonal plates, having no definite meridional arrange- 

 ment, but traversed by five, or fewer, irregular ambulacral 

 grooves radiating from the mouth. And it will be obvious 

 that, in their large number of meridional rows of plates, 

 the Palaeozoic urchins present a much closer resemblance 

 to these extinct Cystoids than is offered by their existing 

 representatives. Indeed, if we be- 

 lieve in the derivation of one form of 

 animal from another, it seems pretty 

 evident that starting from the Cystoids 

 the oldest known Echinoderms we 

 can pass readily into the Palaeozoic 

 urchins, from which we are conducted 

 by the above-mentioned intermediate 

 Secondary forms to species of the type 

 of the common urchin of the present 

 day. What particular advantage the 

 modern urchins have gained by the 

 reduction of their meridional rows to 

 twenty is, however, not very easy to 

 determine ; although this reduction 

 has probably conduced to greater 

 compactness and strength in the 

 structure of their test, which may 

 alone have been a sufficient improve- 

 ment on the older types. 



The transition from the Palaeozoic to the modern forms 

 does not, however, by any means exhaust the modifica- 

 tions which the urchins have undergone with the march 

 of time. Reverting once more to the common urchin 

 (Fig. 57), it should be mentioned that this type, in which 

 the test is radiately symmetrical and the vent and mouth 

 are polar, constitutes the group of the so-called regular 

 urchins. Although this type is still well represented, yet 

 a large number of the urchins of the Secondary, Tertiary, 

 and recent periods have departed very considerably from 



CU 



? r/. 



FIG. 59. ^ide view 

 of the Test of a 

 Palaeozoic Sea-Urchin 

 (Pal&oechiniis). a, 

 ambulacral areas ; ia, 

 intermediate areas. 



