238 COMPARATIVE ANATOMY 



endoplasm near the middle of the body. The endoplasm has 

 much the same character as that of Paramecium, containing 

 food-vacuoleSj pellets, and granules, which move round and 

 round in a regular cyclosis. 



Close alongside of the cytopharynx is a large clear space, 

 which is stationary and generally distinguished from the food- 

 vacuoles by its larger size. This cavity is filled with fluid, and 

 communicates by a very narrow passage with the cytopharynx ; 

 it is a reservoir like that already described in Euglena. Close 

 beside it is a contractile vacuole, whose contents at every con- 

 traction are poured into the reservoir, whence they are slowly 

 emptied into the cytopharynx, and so to the exterior. 



Like Paramecium, Vorticella is in need of a special aperture 

 for the expulsion of undigested matter and solid excreta. This 

 aperture is situated in the cytopharynx close to the opening 

 of the reservoir, and is known as the cytoproct. It is often a 

 permanent opening with a short and narrow tube leading 

 to the endoplasm, but these relations vary very much in 

 different species. 



A meganucleus and a micronucleus are present. The former 

 is very large and elongated. In some species it has the form 

 of a horseshoe, in others it is like a twisted riband. Its shape 

 and position in V. monilata can be better understood from an 

 inspection of fig. 50, than from any description. The micro- 

 nucleus is small, and is placed close alongside of the mega- 

 nucleus in the anterior end of the body. The micronucleus 

 stains with great difficulty, whereas the meganucleus seizes 

 on the ordinary dyes with great avidity and obscures all the 

 structures lying against it. Hence the micronucleus is difficult 

 to see, but it can usually be demonstrated in living specimens, 

 and better still in specimens cleared in glycerine after being 

 killed in a 10% solution of corrosive sublimate to which a few 

 drops of acetic acid have been added. 



Vorticella reproduces itself by binary longitudinal fission. 

 The peristome is contracted, and the body becomes depressed 

 and transversely elongated. The meganucleus is straightened 

 and comes to lie transversely across the middle of the body, 

 forming a fibrillated spindle like that of a Paramecium preparing 

 for division. The micronucleus elongates to form a spindle 

 placed alongside of the meganuclear spindle and both become 

 dumbbell-shaped and divide. (Fig. 50, D.) A constriction 



