VII.] THE BELL-ANIMALCULE. 363 



number of rounded spore-like masses, which are ultimately 

 set free as locomotive germs. 



Sometimes a rounded body, encircled by a basal circlet 

 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 is now known that they are independent 

 individuals, formed as the result of repeated longitudinal 

 fission, 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. There 

 is here a process of " conjugation " sexual in nature, and it 

 is, moreover, preliminary to the budding of the endoplast 

 and the subsequent formation of germs. 



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, the 

 whole process sometimes following that of conjugation and 

 preceding that of germ formation. It does not appear 

 however that this encystment is invariably associated with 

 reproduction ; for the encysted animal after remaining for 

 a longer or shorter time in a temporary condition of rest, 

 may emerge and resume its former state of existence. 



The two genera of Infusoria which most commonly occur 

 in the Frog are Nyctotherus and Balantidium. Both are 

 free and actively locomotive, and the former is particularly 

 remarkable for its relatively large size and semilunar con- 

 tour, and for the length and distinctness of its curved oeso- 

 phagus. Balantidium is pyriform, and has a very short 

 cesophageal depression. 



