C DOBELL 



133 



and also one or more rhythmically contractile vesicles (c.v.) serving 

 probably for excretion or for regulation of the water-content of the 

 protoplasm. Many (jther organs may be present. 



2. The characteristic nuclear apparatus consists of two nuclei or 

 systems of nuclei — a meganucleus (M), and a micronucleus (m). The 

 latter may be represented by one, two, or many nuclei ; whilst the 

 ixieganucleus is frequently segmented, or otherwise modified. 



3. Reproduction is usually effected by transverse fission of the body 

 into two. The meganucleus is divided by a simple constriction, but the 

 micronucleus (or micronuclei, as the case may be) divides mitotically. 

 (See Fig. 1, B.) Unequal bipartition or budding occurs in some forms. 



anil 



".no. 



m. ._ 



/.: 



A 



4. Most ciliates are known to be able to encyst, and so withstand 

 the drying of the liquid in which they live, or other adverse conditions. 

 In some forms temporary cysts are formed during fission. 



5. Most ciliates perform from time to time a peculiar act called 

 conjugation. The details of this process vary considerably in different 

 forms, but the following is an approximately true general account of 

 the events occurring during a typical conjugation'. (See Fig. 2, A — H.) 

 The organisms unite in pairs, the protoplasm subsequently fusing and 

 becoming continuous at the point of contact (Fig. 2, A, B). Each 



1 This account is based on the cases of Colpiditim colpoda (Maupas, 1889) and Para- 

 mecium caudatvm (Calkins and Cull, 1907), which are perhaps as "typical" as any other 

 cases, and are fairly simple because there is only a single micronucleus and meganucleus 

 in these forms. 



