438 THE PROTOZOA 



of a sausage or of a horseshoe, as in Vorticellids (Fig. 183, N), or 

 exhibit the form of a string of beads, as in Stentor (Fig. 8) and 

 Spirostomum (Fig. 180) ; or there may be two macronuclei con- 

 nected by a delicate filament, with a micronucleus beside each, as 

 in Stylonychia ; or, finally, the macronucleus may be broken up 

 to form a diffuse network or a great number of small nuclei. The 

 micronucleus may be single or multiple, but does not vary in form 

 to any marked extent. 



Life-History. Reproduction takes the form of binary fission, 

 usually in the free state ; but some species become encysted prior to 

 division, and then divide into two, four, eight, or a large number 

 of small individuals within the cyst. Binary fission in the free 

 state is, with few exceptions, transverse to the long axis of the 

 body ; but in the fixed, sedentary forms the fission is usually in the 

 vertical plane, or slightly oblique to it, and often takes the form of 

 very unequal fission or budding. In some of the entozoic species 

 of Astomata (p. 439), repeated transverse division of the body 

 without complete separation of the daughter-individuals from one 

 another leads to the formation of chains of individuals, of which 

 the most anterior may be larger than the others. 



As in other Protozoa, colonies may be formed in Ciliata as the 

 result of imperfect separation of sister-individuals produced by 

 fission. This is especially common in the sedentary Peritricha, 

 leading usually to the formation of arborescent growths ; but some- 

 times the colony takes other forms, as, for example, in Ophrydium, 

 where it consists of a great number of individuals embedded in a 

 common mass of jelly which floats freely. 



Encystment is related in various ways to the life-conditions of 

 the Ciliata. Most frequently it appears to take place as a protec- 

 tion against desiccation in free-living forms, or as an adaptation 

 to a change of hosts in parasitic forms. But in some cases it is 

 related to the digestion of food, in others to reproductive processes. 

 In some species it is stated to take place if the supply of food fails, 

 and it can be induced artificially in various ways. 



The process of syngamy has been described above (p. 152, Fig. 77). 

 Summarized, it consists essentially of the following processes ; some 

 exceptions are described below : 



1. Degeneration and ultimate absorption of the macronucleus of 

 each conjugant. 



2. Reducing divisions of the micronucleus to form four micro- 

 nuclei, three of which are absorbed. 



3. Division of the single remaining micronucleus into two pro- 

 nuclei, one stationary, the other migratory. 



4. Passage of the migratory pronucleus of each conjugant across 

 into the body of the other conjugant, where it fuses with the 

 stationary pronucleus. 



