called, it reduces our definition of an animal to the differ- 

 ence indicated in the preceding comparison. 



The Polygastric Infusoria exhibit the next step in the 

 progress of individualizing a higher independent embodi- 

 ment of animal life. A firm central nucleus in which, as in 

 the Gregarina, resides the self-repellent property of spon- 

 taneous division, indicates, however, their essential charac- 

 ter as animated cells. But the granular contents have been 

 developed into secondary cells ; and some of these have 

 combined and coalesced to form special organs, such as 

 cavities for digestion, pulsatile cells for circulating a clear 

 plasmatic fluid, an irritable and contractile integument beset 

 with vibratile cilia : yet a large proportion of the contents 

 of this modified primary cell- wall consists of unchanged 

 secondary cells. 



In the freshwater polype, Hydra, an external layer of cells 

 is partially condensed into an integument, and an internal 

 layer modified to form the gastric secerning villi : in the ten- 

 tacula a greater proportion of the derivative cells have been 

 metamorphosed into the muscular bands, the nodosities, 

 the prehensile darts and tactile cilia. But the chief point 

 that I have now in view is to draw attention to the large 

 proportion of retained and unaltered nucleated cells and 

 nuclei, which are identical in all recognizable characters 

 with the progeny of the primary impregnated germ-cell, 

 and which are ready, therefore, when favourable circum- 

 stances concur, to repeat the acts of assimilation and spon- 

 taneous fission, and, each individually, thus to lay the basis 

 of a new polype. 



A large proportion of the derivative germ-cells is re- 

 tained unchanged in the compound hydriform polypes and 

 in the parenchymatous Entozoa : a smaller proportion in 

 the Acalephae and cavitary Entozoa. Derivative germ- 

 cells are aggregated in the last segment of the Nais, 

 and of the young of other Annelides. We find derivative 

 germ-cells, and masses of nuclei like those resulting from 



