534 ZOOLOGY. 



and the exterior. The mode of propagation of the infusoria 

 has been the object of ranch research, and a great many 

 naturalists think that they may be formed directly by the 

 disintegration of the matters of which leaves, flesh, and other 

 organized bodies are composed ; but this spontaneous genera- 

 tion is far from being sufficiently demonstrated, and it is 

 known that, in certain cases at least, they spring from each 

 other. Moreover, their mode of propagation is quite in 

 accordance with the simplicity of their structure : it is by the 

 spontaneous division of their body into two or more frag- 

 ments, each of which continues to live, and soon becomes a 

 new individual, resembling the first; thus it is that these 

 singular beings in general multiply. 



Their forms are very varied, and they have been divided 

 into several genera, amongst which we may mention the 

 enchelides (in. Fig. 192), which have an oblong body ; the 

 volvoces, which are globular, and continually turn on them- 

 selves ; and the monads (i. Fig. 192), which resemble small 

 points whirling in the water in which they swim. It is owing 

 to the presence of myriads of a particular species of these small 

 monads, whose bodies are coloured red, that salt stagnant 

 waters or ditches acquire a sanguinolent colour. 



CLASS OF THE SPONGIAKIA. 



623. The sponges (Fig. 191) and the other bodies of an 

 analogous structure, only present the more prominent cha- 

 racters of animality during the early period of their life, and 

 resemble later rather unformed vegetables than ordinary 

 animals. At the time of birth, these singular beings suffi- 

 ciently resemble certain infusoria; their body is oval, and 

 provided all over with vibratile cilia, by means of which they 

 swim in the waters ; in this respect the} 1 " bear a resemblance 

 to the larvse of different polyps at the moment when they 

 leave the egg ; but soon the young sponges attach themselves 

 to some foreign body, become almost immovable, give no 

 longer any signs of sensibility or of contractility, and as they 

 grow, become completely deformed. The gelatinous substance 

 of their bodies becomes pierced with holes and canals, tra- 

 versed unceasingly by the waters, and there is developed in 

 their interior a number of horny filaments and spiculse, some- 

 times calcareous, sometimes siliceous, which, disposed in cross 

 bundles .constitute a kind of solid framework. Finally, at 



