16 



ON CLASSIFICATION. 



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

 and the characters which in ay 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. 



The next group to be considered is the division of the Infu- 

 soria; and here, again, within the last few years, prodigious 

 strides have been made in our knowledge of the subject. Al- 

 though the Infusoria have been favourite studies for many vears, 

 still it is only quite recently that the cycle of life of these animals 

 has been made almost completely known, and that we have become 

 acquainted with the true sexual process as it occurs in them. 



Fig. 4. 

 A B c 



j. : ete ./ /r ' .- ' > i -d£» .«-/• t>'.\V- "o^ 



m 



Fig. 4. — Paramoecium bursaria (after Stein) : A, The animal viewed from the dorsal side ; 

 a, cortical layer of the body ; 6, "nucleus;" c, contractile chamber; eld', matters 

 taken in as food ; e, chlorophyll granules. 



B, The animal viewed from the ventral side ; a, depression leading to b, mouth ; 

 c, gullet; d, " nucleus;" d', "nucleolus;" <?, central sarcode. In both these figures 

 the arrows indicate the direction of the circulation of the sarcode. 



C, Paramoecium dividing transversely; a a f , contractile spaces; h b' ', "nucleus" 

 dividing; c <■', "nucleoli." 



The different species of the genus Paramoecium 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 probably that which is 

 common to the whole of the Infusoria, so that an account of the 



