CHAPTER XII. 



OF ECHINODERMATA. 



311. As we ascend the scale of Animal life, we meet with such 

 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 mem- 

 bers of the class Echinodermata ; since even a general account of 

 their complex organization would be quite foreign to the purpose 

 of this work ; whilst 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. 



312. 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 Echinida ; in which it. 

 forms a box-like shell, or "test," composed of numerous polygo- 

 nal 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 inti- 

 mate structure of the shell is everywhere the same ; for it is com- 

 posed of a network, which consists of carbonate of lime with a 

 very small quantity of animal matter as a basis, and which ex- 

 tends in every direction (i. e. in thickness, as well as in length 

 and breadth), its areolce or interspaces freely communicating with 

 each other (Fig. 234). These "areolre," and the solid structure 

 which surrounds them, may bear an extremely variable propor- 



