154 PHYLUM PROTOZOA 



about if it is broken from its stalk. The axial fiber in the stalk is 

 a delicate, striated muscle fiber. Sensation is exercised through 

 the external surface. 



Vorticella reproduces asexually, by a longitudinal division. 

 The process begins at the upper end of the body and proceeds to 

 the base, so that finally there are two individuals upon a single 

 stalk. One of these now separates itself from the stalk, assumes 

 a cylindrical form, and, having developed a band of tempo- 

 rary cilia near one end, swims away to find a place for itself. 

 It soon attaches itself, loses the temporary cilia, and develops 

 a stalk. 



In the case of the colonial Vorticellidae both of the individuals 

 produced by the process of division remain on the stalk. In 

 Zoothamnium the colony is dimorphic ; it contains nutritive in- 

 dividuals which are similar to Vorticella, and reproductive indi- 

 viduals which are large and globose and do not feed. The latter 

 separate themselves from the parents and swim off and found new 

 colonies. This dimorphism and division of labor remind one of the 

 Hydromedusae. In Vorticella, as in Paramecium, reproduction is 

 largely a matter of sufiicient nutrition, well-nourished animals 

 reproducing faster than poorly nourished ones. Conjugation also 

 occurs ; it is brought on by the same conditions as in Paramecium 

 and is highly important to the well-being of the race. The process 

 is, however, somewhat different from conjugation in Paramecium. 

 An individual divides into from two to eight parts. These free 

 themselves from the stalk, acquire each a basal band of cilia, and 

 swim about in the water until they come in contact with indi- 

 viduals of the ordinary kind, with which they fuse. A permanent 

 conjugation is then effected instead of a temporary one as in 

 Paramecium. 



Conjugation, it will be noticed, while it is not a sexual process, 

 is closely allied to such a process, and it is probably through it that 

 sexuality arose in the organic world. In Paramecium and Vor- 

 ticella we have two important steps in the development of sexu- 

 ality. In the former animal the conjugating individuals are of the 

 same size, or isogamous, and the fusion of the two individuals is 



