THE INFUSORIA 



459 



entiation, the bud is set free with a complete or partial coat of 

 cilia as a free-swimming " embryo," " larva," " swarm-spore " 

 (Schwarmer), or " gemmula." The larva often becomes parasitic 

 within the body of another Infusorian (Fig. 192), multiplying there 

 by binary fission. Finally it becomes free again, swims away, 

 attaches itself in a suitable locality, and develops into the adult 

 form. 



The ciliated larvae of acinetans exhibit various types of ciliation, 

 commonly classed as peritrichous (Fig. 193, A, B), holotrichous 

 (Fig. 191. E), and hypotrichous. Collin (882) has studied recently 

 the morphology of the different types of larvae. 



Fro. 192. A and B, Slylonychia mytilus infested by parasitic Acinetans. N, N, 

 Macronuclei of the Stylonychia ; P, parasitic Acinetan embryo from which 

 arise small ciliated larvae (c.e) which swim off and develop into the adult 

 free-living Acinetan. After Stein. 



The most primitive and commonest larval type of larva amongst the 

 Acinetaria is very similar to a free-swimming stage of a Vorticellid. It has 

 a principal axis round which the body is radially symmetrical, with an upper 

 pole (posterior in movement) bearing a rudimentary adoral zone, and a lower 

 (anterior) pole bearing a mass of secretion or a sucker, indicating the future 

 point of fixation and representing the scapula of the Vorticellid ; the body is 

 surrounded by several rings of cilia forming a zone more or less equatorial 

 in position. Such a form, while retaining its radiate symmetry, may become 

 either lengthened or shortened to a remarkable degree in the direction of the 

 principal axis ; in the elongated forms the rings of cilia may increase in 

 number until they cover the whole body, thus producing the holotrichous 

 type. On the other hand, the body may become elongated in the morpho- 

 logically transverse plane, and acquire a bilateral symmetry, with a dorsal 

 surface representing the primitive upper pole and bearing the rudimentary 

 adoral zone, and a ventral surface, with the sucker in the middle of it, repre- 

 senting the primitive lower pole ; secondary anterior and posterior extremities 

 are now distinguishable in relation to progression, but representing opposite 



