432 



THE PROTOZOA 



arranged in meridional rows running from the anterior to the 

 posterior pole, and are of equal length in all parts of the body. 

 An ideally-simple type of this kind is very nearly realized in some 

 of the primitive forms, but as a rule is modified in various ways. 



In the first place, the mouth does not remain anterior, but is 

 shifted to the side of the body, as far as, or even farther than, half- 

 way to the posterior pole (Fig. 181) ; consequently the rows of 

 cilia become displaced from their primitively meridional arrange- 

 ment, and tend to run obliquely round the body. Secondly, a 

 differentiation is set up between the general coat of cilia clothing 

 the body and locomotor in function, and special cilia near or around 

 the mouth, which are usually much longer than the others, and 

 modified in various ways in connection with the function of food- 

 capture. The mouth itself becomes surrounded by a special area 



termed the " peristome," in 

 which are found the special 

 food-capturing cilia. 



In forms of creeping habit 

 the form becomes still more 

 modified. The body becomes 

 flattened, and a ventral sur- 

 face, turned towards the 

 substratum and bearing the 

 mouth and peristome, is 

 distinguisKfed from the oppo- 

 site or dorsal surface. Even 

 more marked are the adapta- 

 tions of the coat of cilia to 

 this mode of life (Fig. 182). 

 The locomotor cilia become 



restricted to the ventral surface, and those on the dorsal side either 

 tend to disappear altogether or persist with a purely tactile function. 

 The cilia of the ventral surface tend to form tufts which fuse into 

 cirri (p. 55), with which the animal creeps as if on legs. 



Sedentary forms may be attached temporarily by means of special 

 cilia or adhesive organs, or more or less permanently by a portion 

 of the body-surface on the side opposite to the mouth. In such 

 forms (Fig. 183) the general coating of cilia may be retained, or 

 may disappear entirely, only the peristomial cilia persisting ; but 

 locomotor cilia may be developed temporarily, enabling the animal 

 to become detached from one spot, and to swim away and attach 

 itself again elsewhere. In sedentary forms the point of attachment 

 may be drawn out into a stalk, which may be of great length rela- 

 tively, and may be a secreted structure or a portion of the body 

 drawn out. In the second case the stalk may contain highly-per- 



ABC 



FIG. 181. Diagram illustrating the shifting 

 of the mouth, and the consequent displace- 

 ment of the rows of cilia, in Ciliata, from a 

 form in which the mouth is at the anterior 

 pole and the rows of cilia run a meridional 

 course (A), to a form in which the mouth is 

 shifted to the side of the body (G). After 

 Delage and Hcruuard. 



