174 UNICELLULAR ANIMALS. 



sion of higher differentiation. The body is pear-shaped or coni- 

 cal, attached at its apex by a long slender stalk. The latter 

 consists of a slender contractile axial filatnent^ by means of 

 which the stalk may be thrown into a spiral and the body drawn 

 down, and an elastic sheath (continuous with the general cuticle) 

 by which the stalk is straightened (Fig. 90). The cilia are con- 

 fined to a thickened rim, the peristorne (^:>), surrounding the 

 base of the cone, which may be termed the dislc. At one side 

 the disk is raised, forming a projecting angle covered with cilia, 

 and known as the epistome (ej)). At the same side the peristome 

 dips downw«ards, leaving a space between it and the ej)istome. 

 This space is the vestibule (v), and into it the mouth opens. In 

 it likewise is situated an anal spot like that of I^aramcechmi. 

 The cilia produce a powerful vortex centering in the mouth, by 

 means of which food is secured. The macronucleus {inac) is 

 long, slender, and horseshoe-shaped ; the small spherical micro- 

 nucleus (mic) lies near its middle portion. There is usually 

 but one contractile vacuole. 



Yorticella multiplies by fission, division of the protoplasm 

 being accompanied by that of the macronucleus and micronu- 

 cleus (Fig. 91). The plane of fission is vertical (thus dividing 

 the peristome into halves), but extends only through the main 

 body, leaving the stalk undivided. At the close of the process, 

 therefore, the stalk bears two heads. One of these remains 

 attached to the original stalk, while the other folds in its peri- 

 stome, acquires a second belt of cilia around its middle (Fig. 91), 

 breaks loose from the stem, and s\dms actively about as the so- 

 called " motile form." Ultimately it attaches itself by the base, 

 loses its second belt of cilia, develops a stalk, and assumes the 

 ordinary form. By this process dispersal of the species is en- 

 sured. Under unfavorable conditions similar motile forms are 

 often produced wdtliout previous fission, the head simply acquir- 

 ing a second belt of cilia, dropping off, and swimming away to 

 seek more favorable surroundings. Yorticella may become en- 

 cysted, losing its peristome and mouth, becoming rounded in 

 form, acquiring a thick membrane, and having no stalk. In 

 this state it is said sometimes to multiply by endogenous dimsion^ 

 breaking up into a considerable number of minute rounded 

 bodies (spores) each of which contains a fragment of the 



