OF ANIMALCULES. 19 



greatest imaginable rapidity, darting, leaping, or swim- 

 ming, others merely creep or glide along ; and many 

 are altogether so passive that it requires long and patient 

 observation to discover any of their movements at all. 

 One description are perceptibly soft, and yield easily to 

 the touch ; another are covered with a delicate shell or 

 horn-like coat. Of the latter order there are different 

 degrees of density, as in the Volvox, Gonium, &c. where 

 the envelope is comparatively thick ; and where, strange 

 to say, the internal substance separates by the mode of pro- 

 pagation into several portions, forming so many distinct 

 young ones, which at their birth burst the envelope, and 

 the parent becomes entirely dissipated. In others of 

 this order the shell is merely a plate covering the 

 body, resembling that of the tortoise : sometimes it 

 includes the body, so as to leave only two small aper- 

 tures at the extremities, and at others it is bivalve, and 

 encloses the creature like that of the oyster or muscle. 



A reference to the Plates, also, will convey a pretty 

 accurate notion of the extraordinary methods of propa- 

 gation with animalcules. All vertebrated animals are 

 either oviparous or viviparous, which terms sufficiently 

 designate their modes of production : but it is not so 

 with animalcules ; for, in addition to these two methods — 

 1. Animalcules propagate by a spontaneous scissure, or 

 division of their bodies into tw^o or more portions, each 

 one forming a new creature, which, on its arrival at ma- 

 turity, pursues the same course. These divisions take 



