THE LIFE OF THE SIMPLEST ANIMALS 13 



The body is pear-shaped or bell-shaped, with a mouth 

 opening at the broad end, and a delicate stem at the 

 narrow end. This stem is either hard and stiff, or is 

 flexible and capable of being suddenly contracted in a 

 close spiral. In the body mass there is one pulsating 

 vacuole and one nucleus. Usually many Vorticellce are 

 found together on a common stalk, thus forming a proto- 

 zoan colony. 



The life processes of Vorticella are of the simple kind 

 already observed in Amoeba and Paramoecium. Vorticella 

 shows, however, some modifications of the process of repro- 

 duction which are interesting. The plane of division of 

 Vorticella is parallel to the long axis of the pear-shaped 

 body, so that when fission is complete there are two Vorti- 

 cella on a single stalk. One of the two becomes detached, 

 and by means of a circle of fine hairs or cilia which appear 

 around its basal end leads a free swimming life for a short 

 time. Finally it settles down and develops a stalk. Vorti- 

 cella shows two kinds of fission one the usual division 

 into equal parts, and another division into unequal parts. 

 In this latter kind, called reproduction or multiplication 

 by budding, a small part of the parent body separates, 

 develops a basal circle of cilia, and swims away. The pro- 

 cess of conjugation also takes place among the Vorti- 

 cella, but they are never two equal forms which conju- 

 gate, but always one of the ordinary stalked forms and 

 one of the small free - swimming forms produced by 

 budding. 



Here, then, in the life of Vorticella, are new modifica- 

 tions of the life processes ; but, after all, these life processes 

 are very simply performed, and the body is like the body of 

 the Amoeba, a single cell. Vorticella. is plainly one of "the 

 simplest animals." 



7. Gregarina. A fourth kind of protozoan to which we 

 can profitably give some special attention is Gregarina 

 (Fig. 9), the various species of which live in the alimentary 



