362 ELEMENTARY BIOLOGY. [CHAP. 



as a spheroidal drop, henceforward fre^ in the soft central 

 substance. Such being the mode of formation of the vesicles, 

 they have been termed vesicles of ingestion; in some Bell- 

 animalcules, they are carried round with the deeper layer of 

 protoplasm or endosarc in a movement of circulation, 

 passing up one side of the body, 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 out of the body, surrounded as a rule by a watery 

 vacuole or vesicle of egestion^ This process takes place by 

 an aperture leading into the vestibule, which exists only at 

 the moment of extrusion of the faeces, and is indistinguish- 

 able 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 e^doplast. 

 It is elongated and bent upon itself into a afescentic or 

 horseshoe shape. 



The numerous species and varieties of bell-animalcules 

 are, for the most part, colourless. Green varieties are how- 

 ever occasionally to be met with, the green colour being 

 due to a deposit of chlorophyll within the endosarc, com- 

 parable to that seen within the endoderm of the green- 

 hydra. This colouring matter may be restricted to small 

 bodies or chloroplastids identical with those of the polype 

 referred to, or diffused throughout the cell-protoplasm (en- 

 dosarc) in a manner such as, there is good reason to believe, 

 is never the case in the vegetable organism. 



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 getmnation from the 

 endoplast, in which latter case the endoplast divides into a 



