THE INFUSORIA 189 



sary. Ciliates without micronuclei, and hence possibly without 

 conjugation or endomixis, are also known. It therefore appears 

 that such rejuvenescence as may be necessary in these protozoa 

 can be accomplished by more than one means. The one factor in 

 conjugation that is unquestionable is the union of two diverse lines 

 of descent; for, just as the fertilized egg of a many-celled animal 

 is a single cell derived by union of two different germ cells (Fig. 

 116, p. 233), so the ex-conjugants of paramcecium are cells of 

 double parentage. Thus, the basis for bi-parental inheritance 

 exists among Infusoria as in other protozoa. Conjugation in the 

 ciliated protozoa, and presumably in other unicellular forms, is, 

 therefore, significant (1) as a means of uniting two diverse lines of 

 descent with the result that variation may be increased; and (2) 

 apparentl}^, under some conditions, as a means of stimulating cell 

 division and other cell functions and hence rejuvenating lines that 

 might otherwise perish. 



Other Infusoria. — Since the cihated Infusoria, or CUiata, 

 include most of the species of Protozoa that are likely to be seen 

 by the student, it will be useful to indicate the more important 

 subdivisions and the names of representative genera. Ciliata 

 are classified, according to the nature and distribution of their 

 cilia, into the following groups: 



I. Aspirigera. — Forms without a spiral zone of oral ciHa 

 or membranelles. 



1. Order, Holotricha. — Having the cilia of about 

 equal length and evenly distributed in the simpler 

 forms; and with cilia having more complex 

 arrangements in the more specialized forms. With 

 a cj^ostome, except in some of the parasitic types. 

 Opalina, Prorodon, Coleps, Didinium, Amyhilej)- 

 tus, Lionotus, Loxodes, Dileptus, Colpoda, Fron- 

 tonia, and Paramcecium are among the genera of 

 this order that are common in fresh water 

 {cf. Fig. 94). 



II. Spirigera. — Forms with a conspicuous spiral zone 

 of larger cilia or vibratile membranelles leading to the 

 mouth. 



2. Order, Heterotricha. — Usually swimming, some- 

 times attached. Body cilia small or reduced in 



