2 CLASSIFICATION OF THE ANIMAL KINGDOM. 



form definite " tissues." The Protozoa and Metazoa agree, therefore, with 

 one another in the fact that they are, to begin with, simple undivided 

 masses of protoplasm, but they differ in the results produced by the 

 development of this protoplasm. In the case of the Protozoa, the original 

 mass of protoplasm may remain undifferentiated, or it may develop a 

 " nucleus," and become thus a " cell ; " but it does not become converted 

 into a complex structure composed of metamorphosed cells or " tissues." 

 On the other hand, in the Metazoa, the original mass of protoplasm is not 

 only always a true cell, but it becomes converted by a process of regular 

 division into a primitive aggregate of cells, and these secondary cells 

 become finally differentiated into the complex "tissues" of which the 

 body of the adult is composed. 



The principal difficulty in the way of accepting this primary, and in 

 the main natural, division of the animal kingdom, is afforded by the 

 Sponges, which are morphologically Protozoa, while, according to the 

 views of many naturalists, they are developmentally Metazoa. That is 

 to say, they present in their morphological elements so close a resemblance 

 to certain of the Protozoa that we can hardly doubt of their close genetic 

 connection with the latter ; while, on the other hand, they exhibit in 

 their development (as this has been usually interpreted) the " segmen- 

 tation " of the primitive ovular cell which is characteristic of the Metazoa. 

 Whether or not they possess any definite internal vacuity which can be 

 properly compared with the " body-cavity " of the normal Metazoa may 

 still legitimately remain a matter for doubt. It is also still a matter of 

 reasonable doubt whether the development of the Sponges is really pro- 

 perly comparable to that of the Metazoa ; and, if we accept the views of 

 Mr Saville Kent upon this subject, it certainly is not so. It should also 

 be borne in mind that there are certain of the Protozoa (e.g., some of the 

 Radiolaria) in which it is not possible to absolutely assert that the adult 

 is unicellular. 



As regards the primary divisions of the Protozoa, it has not been un- 

 usual to accept the presence of a permanent mouth, or ingestive aperture, 

 as a good mark of distinction ; and, in accordance with this, the Protozoa 

 have been divided into the two primary sections of the Astomata (com- 

 prising the Gregarinida and Rhizopoda), and the mouth-bearing forms, or 

 Stomatoda (comprising the Infusoria). Many of the Infusoria, however, 

 do not possess a mouth in the proper sense of the term ; and Mr Saville 

 Kent has recently (' Manual of Infusoria ') proposed the following classi- 

 fication of the Protozoa, based upon a more accurate interpretation of the 

 methods in which the ingestion of food is effected by different members 

 of the sub-kingdom : 



SECTION A, PANTOSTOMATA. Ingestive area diffuse. This section 

 comprises the Gfregarinida and the most typical forms of the Ehizo- 

 poda (viz., the Amcebea, Monera, Foraminifera, and Radiolaria.) 



SECTION B, DISCOSTOMATA. Ingestive area discoidal, not constitut- 



