44 INFUSORIA, 



(90.) When examined with a good microscope, the Infusoria are 

 found to be possessed of another organ of very mysterious character, 

 which, in accordance with the prevailing notion of their analogy with 

 animal or vegetable cells, has been named the nucleus, and which cer- 

 tainly, as we shall see hereafter, seems to play an important part in 

 their economy, more especially in connexion with the propagation of 

 these animalcules. 



The nucleus is usually roundish or longish, or even (as in many Vor- 

 ticellince and Stentor) much elongated and band-like. It is enveloped in 

 a peculiar membrane, and generally presents a homogeneous or finely 

 granular appearance. It appears constantly to enclose a cavity sur- 

 rounded with very thick walls, and sometimes contains a smaller body, 

 designated nucleolus, which, however, in certain species is situated ex- 

 ternal to the nucleus. 



The nucleus, as a general rule, seems to be affixed to the walls of the 

 animalcule, as it does not appear at all to participate in the rotatory 

 movements observable in the interior of the body. 



(91.) In most, if not all Infusoria, the whole surface of the body is 

 capable of exuding a gelatinous matter. Some do this regularly ; and 

 the exuded material either retains its gelatinous consistence, or, hard- 

 ening into a horny substance, forms a sheath or shell (lorica), into 

 which the animalcule can contract itself more or less completely. 



Another kind of exudation also occurs in a great many species, which 

 leads to the formation of a case or cyst enclosing the animalcule that 

 secretes it the object of which appears to be the protection of the 

 encysted animal from unfavourable circumstances in the surrounding 

 water, and from death from desiccation. This process of encystation is 

 sometimes connected with the reproductive function, as we shall see 

 hereafter. 



(92.) The reproduction of these animalcules is effected in various 

 ways ; and not unfrequently the same individual would appear to pro- 

 pagate by two or three different modes of increase. 



(93.) The first is by external gemmules or buds, which sprout like 

 minute gelatinous tubercles from the surface of the body, and, gradually 

 attaining the shape of their parent, develope the cilia characteristic of 

 their species, and soon become independent beings, although they do not 

 attain to their full growth until some time after their separation. 



(94.) The most usual mode of propagation, however, is by sponta- 

 neous fissure, or division of the body of an adult animalcule into two or 

 more portions, each of which is perfect in all its parts. This singular 

 kind of generation, by which the old animalcule literally becomes con- 

 verted into two or more young ones, is accomplished in various ways, 

 which will require separate notice. 



(95.) In the oval forms of the Infusoria, the line of separation gene- 

 rally divides the body transversely into two equal portions, by a process 



