REPRODUCTION OF CILIATA. 



201 



to the shape of a horse-shoe or a band, and may be broken up into 

 a number of fragments. It contains a granular viscid substance, 

 is bounded by a delicate membrane, and, a b 



according to the erroneous views of Stein 

 and Balbiani, gives rise to ova or to germi- 

 nal spores. The nucleolus or paranucleus 

 also varies in form, position, and number 

 in different species. It is always much 

 smaller than the nucleus, and is strongly 

 refractile ; it usually lies close to the 

 nucleus, or even sunk in a cavity of the 

 latter. Both play an important part in 

 the reproduction of the Infusoria. 



The most usual method of reproduction in the Infusoria is by 

 fission. When the forms reproduced remain together and connected 

 with the parent, a colony of Infusoria is formed, e.g., the stocks of 

 Epistylis and Carchesium. Fission usually takes place by a trans- 

 verse division (at right angles to the long axis), as in the Oxytrichidce, 



FIG. 141. a, Asp'ullsca Ij/ncaster 

 (after Stein). b,Aspidiscapolyity- 



la, during fission (after Stein). 



FIG. 112. PodopJirya yemmipara (after R. Hertwig). a, with extended suction-tubes ;md pre- 

 hensile tentacles, with two contractile vacuoles. b, the same with ripe buds, iu which 

 processes of the branched nucleus N enter, c, free young form. 



Stentoridoe, etc., and, obeying definite laws, follows conjugation and 

 division on the one hand of the nuclei, and on the other of the 

 nucleoli (fig. 141). Less frequently (Vorticetta) the fission takes 

 place through the long axis (fig. 143, a, l>), and far more rarely in 

 a diagonal direction. The asexual reproduction is often preceded 

 by encystment, which appears to be of great importance for the 



