4 6 



SYSTEMATIC ZOOLOGY 



plays the most important role in conjugation, and in some cases 

 the conjugation is permanent, i.e. the two individuals com- 

 pletely fuse with one another. The division of the class into 

 orders depends chiefly on differences in the size and distribution 



of the cilia. 



Order i. Holotricha 



The Holotricha (Gr. oXos, whole, and Opi%, a hair) differ from 

 the other orders of the Ciliata in that they have cilia of nearly 



c.vac 



uc.gr 



C.rar 



FlG. 28. Paramcecium caudatum. A, the appearance of the living 

 animal from the oral surface; B, the same individual in optical sec- 

 tion : the arrows indicate the course of the food particles, buc. 

 gr, buccal groove; corf, ectosarc, containing trichocysts; cu, cuticula, 

 with projecting cilia ; c.vac, contractile vacuole, f.vac, food vacuole; 

 gul, gullet or oesophagus; vied, entosarc ; mth, mouth; nu, macro- 

 nucleus; pa. nit, micronucleus ; trch, discharged trichocysts. (From 

 Parker's Biology. ) 



equal length distributed over the entire body. Paramcecium 

 (Fig. 28) is a common example of this order ; it is very abundant, 

 occurring in hay infusions and in stagnant water. 



Order 2. Heterotricha 



The Heterotricha (Gr. erepo?, different, and 0p<'£, a hair) have 

 cilia over the whole surface of the body, but in the region of the 



