38 COMPARATIVE ANATOMY CHAP. 



the animal kingdom the more sharply defined and the more thorough 

 is the division of labour. The epithelial areas no longer consist of 

 similar cells, but the cells of the same epithelium, having different 

 functions to perform, are differently constructed. 



The comparison of the adaptations by which different cells 

 (especially epithelial cells) of the Metazoa seem to be suited for definite 

 functions, with the adaptations we have described among the Protozoa, 

 is very instructive. In the latter, in the most complicated cases, 

 different portions of one and the same cell appear specially adapted 

 for the performance of definite functions. Among the Metazoa, how- 

 ever, in a number of tissue cells one of the many adaptations of a 

 complicated unicellular Protozoon becomes the principal adaptation 

 suppressing all or most of the others, or totally obliterating them; 

 in this way cells suited for the performance of one special function 

 arise. We find among the Protozoa as adaptations for movement, 

 for taking in food, and for respiration cilia, flagella, and so on. 

 Epithelial cells of Metazoa very often assume such adaptations as the 

 most prominent characteristic of the cells. Either all cells of an 

 epithelium are covered with cilia then we speak of ciliated 

 epithelium or only groups of cells or single cells are thus covered. 

 The cilia of a cell may be replaced by a flagellum, e.g. in the 

 endoderm of the sponges. Then the epithelial cells so provided are 

 called flagellate cells, which often strongly remind one of the 

 Choanoflagellata. 



These cilia and flagella serve among the Metazoa, as among the 

 Protozoa, for very various purposes. 



1. They cause locomotion in Metazoa of small size which live in 

 water (cilia of the body epithelium of a few fihabdoccelidce, ciliated 

 plates of the Ctenophora, cilia of the wheel organ of some Rotifera\ 

 and especially in the free swimming larval stages of many Metazoa 

 (general ciliation of many larvse, ciliated rings and bands of the larvae 

 of Platodes, Vermes, Molluscs, and Echinoderms). 



2. They serve for whirling food within reach, as they surround 

 the oral aperture, e.g. Rotatoria. 



3. They continually cause new digestible material to pass over the 

 digesting epithelium ; in many cases they at the same time cause in 

 the enteric cavity a constant renovation of water and of the food 

 suspended in it. Intestinal respiration. Ciliated epithelium of the 

 intestinal canal. 



4. They constantly bring fresh oxygenated water into contact with 

 the epithelium. Respiration. Ciliated epithelium of the gills. 



5. If they carry on these activities at certain parts of the 

 epithelium where sensory nerves come to the surface, they subserve 

 special functions of sensation. Ciliated grooves. Olfactory pits. 



Very many Protozoa use pseudopodia and amoeboid processes for 

 locomotion and ingestion of food, and also perhaps for respiration. In 

 many Metazoa the taking of nutritive particles into the intestinal 



