THE INFUSORIA. 17 



way grow up into SpongiUse like that from which they pro- 

 ceeded. 



But there is, in addition, a true sexual process, which goes 

 on during the summer months. Individual sponge particles 

 become quiescent, and take on the character of ova ; while, in 

 other parts, particular sponge particles fill with granules, the 

 latter eventually becoming converted into spermatozoa. 



These sacs burst and some of the spermatozoa, coming into 

 contact with the ova, impregnate them. The ova develop and 

 grow into ciliated germs (D, Fig. 4), which make their way out, 

 and, after swimming about for a while, settle themselves down 

 and grow up into Spongillse. 



Now that we know the whole cycle of the life of the sponges, 

 and the characters which may be demonstrated to be common 

 to the whole of this important and remarkable class, I do not 

 think any one who is acquainted with the organization or the 

 functions of plants will be inclined to admit that the Spongida 

 have the slightest real affinity with any division of the vegetable 

 kingdom. 



V. THE INFUSORIA. 



Although the Infusoria have been favourite studies for many 

 years, it is only quite recently that the anatomy of these 

 animals has been satisfactorily made out. 



The different species of the infusorial genus Paramoeeium are 

 very common among the microscopic inhabitants of our fresh 

 waters, swimming about by means of the vibratile cilia with 

 which the whole surface of their bodies is covered ; and the 

 structure which essentially characterises these animals is pro- 

 bably that which is common to the whole of the Infusoria, so 

 that an account of the leading structural features of Paramcecium 

 is, in effect, a definition of those of the group. 



Imagine a delicate, slipper-shaped body inclosed \vithin a 

 structureless membrane, or cuticula, which is formed as an excre- 

 tion upon its outer surface. At one point (Fig. 5, B a) the body 

 exhibits a slight depression, leading into a sort of little funnel 

 (6 c) coated by a continuation of the same cuticular investment, 



c 



