GENERAL ZOOLOGICAL INTEREST. in 



Relics of siliceous Radiolarian shells are also known from Silurian 

 strata onwards, with, perhaps, the exception of the Devonian. Best 

 known are those which form the later Tertiary deposits of Barbados 

 earth, from which Ehrenberg described no fewer than two hundred and 

 seventy-eight species. 



General zoological interest. — The Protozoa illustrate, in 

 free and single life, forms and functions like those of the 

 cells which compose the many-celled animals. Typically, 

 they show great structural or morphological simplicity, but 

 great physiological complexity. Within its single cell the 

 Protozoon discharges all the usual functions, while in a 

 higher animal distinct sets of cells have been specialised for 

 various activities, and each cell has usually one function 

 dominant over the others. The Metazoan cells, in acquiring 

 an increased power of doing one thing, have lost the 

 Protozoan power of doing many things. 



The Protozoa remain at the level represented by the 

 reproductive cells of higher forms, and are comparable to 

 reproductive cells which have not formed bodies. In the 

 sexual colonies of Volvox, however, w e see the beginning of 

 that difference between reproductive cells and body cells 

 which has become so characteristic of Metazoa. The 

 Protozoa are self-recuperative, and in normal conditions 

 they are not so liable to " natural death " as are many-celled 

 animals. Weismann and others maintain that they are 

 physically immortal. 



They illustrate — (a) the beginnings of reproduction, from 

 mere breakage to definite division, either into two, as in 

 fission, or in limited time and space into many units, as in 

 the formation of spores within a cyst ; (&) the beginnings of 

 fertilisation, from " the flowing together of exhausted cells " 

 and multiple conjugation, to the specialised sexual union of 

 some Infusorians, where two individuals become closely 

 united ; (c) the beginnings of sex, in the difference of size 

 and of constitution sometimes observed between two con- 

 jugating units ; (d) the beginnings of many-celled animals in 

 the associated groups or colonies which occur in several of 

 the Protozoan classes. These colonies show a gradation in 

 complexity. Raphidiophrys and other Heliozoa form loose 

 colonies, which arise by the want of separation of the 

 products of fission. Among the Radiolarians there are 



