884 



CHAPTER XYI 



ECHINODEBMA 



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 oneself 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 Echino- 

 derma, of whose complex organisation even a general account 

 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 ; while the study of their embryonic forms, which can be pro- 

 secuted by any seaside observer, brings into view an order of facts 

 of the highest scientific interest. 



It is in the structure of that calcareous skeleton which exists 

 under some form in nearly every member of this class that the ordi- 

 nary microscopist finds most to interest him. This attains its highest 

 development in the Echinoidea, 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 l 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 areolce or interspaces freely communicating with each 

 other (figs. 671, 672). 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 predominate ; and. the texture may have either a re- 

 markable lightness and porosity, if the network be a very open one, 

 like that of fig. 671, or may possess a considerable degree of com- 

 pactness, if the solid portion be strengthened. Generally speaking, 

 the different layers of this network, which are connected together 

 by pillars that pass from one to the other in a direction perpendicu- 



