IX.] THE BELL-ANIMALCULE. 89 



aperture which exists only at the moment of extrusion of the 

 fseces, 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 horse- 

 shoe 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 possessed 

 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 loco- 

 motive 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 " con- 

 jugation" 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 sur- 

 rounds 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 state 

 of existence. In thus passing into a temporary condition 

 of rest many of the other Infusoria resemble Vorticella. 



The two genera of Infusoria which most commonly occur 

 in the Frog are Nyctotherus and Balantidium. Both are free 



