555 



CHAPTER XII. 



ECHINODERMATA. 



429. As we ascend the scale of Animal life, we meet with snch 

 a rapid advance in complexity of structure, that it is no longer 

 possible to acquaint one's-self with any organism by Microscopic 

 examination of it as a whole ; and the dissection or analysis which 

 becomes necessary, in order that each separate part may be studied 

 in detail, belongs rather to the Comparative Anatomist than to 

 the ordinary Microscopist. This is especially the case with the 

 Echinus (Sea-Urchin), Asterias (Star-fish), and other members of 

 the class Echinodermata, even a general account of whose com- 

 plex organization would be quite foreign to the purpose of this 

 work. Yet there are certain parts of their structure which furnish 

 Microscopic objects of such beauty and interest that they cannot 

 by any means be passed by ; besides which, recent observations on 

 their Embryonic forms have revealed a most unexpected order of 

 facts, the extension and verification of which will be of the greatest 

 service to science, — a service that can only be effectually rendered 

 by well-directed Microscopic research in fitting localities. 



430. It is in the structure of that Calcareous Skeleton which 

 probably exists under some form or other in every member of this 

 class, that the Microscopist finds most to interest him. This 

 attains its highest development in the EcMnida ; in which it 

 forms a box-like Shell or ' Test, ' composed of numerous polygonal 

 plates jointed to each other with great exactness, and beset on its 

 external surface with Spines, which may have the form of prickles 

 of no great length, or may be stout club-shaped bodies, or, again, 

 may be very long and slender rods. The intimate structure of the 

 Shell is everywhere the same ; for it is composed of a network, 

 which consists of Carbonate of Lime with a very small quantity of 

 Animal matter as a basis, and which extends in every direction 

 (i.e., in thickness as well as in length and breadth), its Areola 

 or interspaces freely communicating with each other (Fig. 286). 

 These ' Areolae,' and the solid structure which surrounds them, 

 may bear an extremely variable proportion one to the other ; so 

 that in two masses of equal size, the one or the other may greatly 



