20 INTRODUCTION TO CLASSIFICATION. 



containing spaces are stomachs ; for it is impossible to imagine 

 any kind of anatomical arrangement which shall permit true 

 dilatations of an alimentary canal to rotate in any such manner. 

 Faecal matters are extruded from an anus, which is situated not 

 far from the mouth, but is invisible when not in use. It is an 

 interesting and important character of the Infusoria, in general, 

 that, under some circumstances, they become quiescent and 

 throw out a structureless cyst around their bodies. The Infu- 

 sorium then not unfrequently divides and subdivides, and, the 

 cyst bursting, gives rise to a number of separate Infusoria. 



The remarkable powers of multiplication by fission which 

 many of the group exhibit are well known ; but within the last 

 few years the investigations of Miiller, Balbiani, Stein, and 

 others, have led them to believe that these minute creatures 

 possess a true^ process of sexual multiplication, and that the 

 sexual organs are those which have been denominated " nucleus " 

 and "nucleolus." The nucleus is considered to be the true 

 ovary the nucleolus, the testis, in Paramcecium. But further 

 information is required before this interpretation can be finally 

 accepted. All that can be said to be made out with perfect cer- 

 tainty is, that occasionally vibrio-like rods (the supposed sper- 

 matozoa) are seen in the enlarged and modified nucleolus and 

 nucleus ; and that the nucleus may under certain circum- 

 stances give rise to germs by fission. 



A process of conjugation has been observed in many Infusoria. 



In giving an account of the preceding groups, I have substi- 

 tuted for a definition of each class a description of the structure 

 of some particular member of that class, or of the organic 

 features which are most obviously characteristic of the class ; 

 because, in hardly any of those groups has the structure of many, 

 and widely different, members been thoroughly and exhaustively 

 worked out. 



I entertain little doubt, however, that the main features of 

 the description of Spongilla might substantially be taken as a 

 definition of the Spongida, and those of the description of Para- 

 mcecium, as a definition of the Infusoria. On the other hand, 

 we possess no such complete knowledge of the vital cycle of 



