9z ELEMENTARY BIOLOGY. [CHAP. 



then crossing over below the disc and descending on the 

 other side. Sooner or later the contents of these vesicles 

 are digested, and the refuse is thrown into the vestibule by 

 an aperture which exists only at the moment of extrusion of 

 the faeces, and is indistinguishable at any other time. 



A portion of the substance of the body, which is slightly 

 different in transparency and in its reactions to colouring 

 substances from the rest, is called the nucleus or endoplast. 

 It is elongated and bent upon itself into a crescentic or 

 horseshoe shape. 



The Bell-animalcules multiply in two ways; partly by 

 longitudinal fission, when a bell becomes cloven down the 

 middle, each half acquiring the structure previously pos- 

 sessed by the whole ; and partly by gemmation from the 

 endoplast, in which latter case the endoplast divides and one 

 or more of the rounded masses thus separated are set free 

 as locomotive germs. 



Sometimes a rounded body, encircled by a ring of cilia 

 but having otherwise the characters of a Vorticella bell, is 

 seen to be attached to the base of the bell of an ordinary 

 Vorticella. It was formerly supposed that these were buds, 

 but it appears that they are independent individuals, which 

 have attached themselves to that to which they adhere and 

 are gradually becoming fused with it, so that the two will 

 form one indistinguishable whole. It is probable that this 

 " conjugation" has relation to a sexual process. 



Under certain circumstances a Vorticella may become 

 encysted. The peristome closes and the bell becomes con- 

 verted into a spheroidal body, in which only the nucleus 

 and the contractile vesicle remain distinguishable. This 

 surrounds itself with a structureless envelope or cyst, from 

 which, after remaining at rest for a longer or shorter time, 

 the Bell-animalcule may emerge and resume its former 



