14-0 THE MICROSCOPE AND ITS KEVELATIONS. 



CHAPTER XIV. 

 ECHINODERMATA. 



531. 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 members of the class Echinodermata; even a general account 

 of whose complex 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; while the study of their Embryonic forms, which 

 can be prosecuted by any Sea-side observer, brings into view an order of 

 facts of the highest scientific interest. 



532. It is in the structure of that Calcareous Skeleton which proba- 

 bly exists under some form in every member of this class, that the ordi- 

 nary 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 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 inti- 

 mate 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 quan- 

 tity 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 inter- 

 spaces freely communicating with each other (Figs, 366, 367). These 

 'areolae/ and the solid structure which surrounds them, may bear an ex- 

 tremely variable proportion one to the other; so that in two masses of 

 equal size, the one or the other may greatly predominate; and the tex- 

 ture may have either a remarkable lightness and porosity, if the network 

 be a very open one like that of Fig. 366, or may possess a considerable 

 degree of compactness, if the solid portion be strengthened. Generally 

 speaking, the different layers of this network, which are connected to- 



f ether by pillars that pass from one to the other in a direction perpen- 

 icular to their plane, are so arranged that the perforations in one shall 

 correspond to the intermediate solid structure in the next; and their 

 transparence is such that when we are examining a section thin enough 

 to contain only two or three such layers, it is easy, by properly focussing 

 the Microscope, to bring either one of them into distinct view. From 

 this very simple but very beautiful arrangement, it comes to pass that 

 the plates of which the entire ' test ' is made-up possess a very consider- 



