DIATOMACE^l. 



305 



would be more correctly placed in a median, orMolluscian 

 sub-kingdom. Appearing everywhere with the first born of 

 life, and wherever matter is found in a condition fit for 

 their development and nourishment, these marvellous 

 little siliceous creatures have been preserved and brought 

 down to us in forms unchanged, from the remotest periods 

 of our globe's history ; and supplying, as they do to the 

 microscopist, some of the most valuable test-objects, the 

 Gyrosigma, Grammatophora, Fragilaria, Rhipidophora, 

 Pleurosigma angulatum, and many others ; it cannot be 

 a matter of surprise, that considerable interest should 

 have been awakened in them. 



Fig. 178. 



1, Navicula. 2, Side view of the same, showing apparent arrangement of cilia 

 and sarcode of animal. 3, Pleurosiqmata lanceolatum. 4, Lateral view of a 

 portion oi the same. 5, Synedra. 6, A portion of Pinnularia. 7, Pinnularia. 

 8. Stauroneis. 



Most of these little animals inhabit the waters of the 

 earth, and are always met with in greatest profusion in 

 places where Confervce and other forms of aquatic vegeta- 

 tion are abundant. In these creatures, as in Rhizopoda, 

 we find, in addition to the dark nucleus, one or more 

 clear spaces which expand and contract alternately. These 

 spaces are usually round, and sometimes exist in such 

 numbers as to constitute a continuous chain-like vessel ; and 

 it seems probable, that in these spaces we have the first 

 rudiments of a circulating system, which attains to a more 



