4G2 infusoria : ENCYSTING process. 



great rapidity, so as to describe a sort of inverted cone, whereby 

 a current is brought towards the mouth. This latter form has been 

 described by Prof. Ehrenberg under the name of Aspidisca. It 

 is very much smaller than the larva ; the difference being, in fact, 

 twice as great as that which exists between a and p, q (Fig. 242), 

 since the last two figures are drawn under a magnifying power 

 twice as great as that employed for the preceding. How the Aspi- 

 disca-fovm. in its turn gives origin to the Oxytricha-iorm, has not 

 yet been made-out. A Sexual process, it may be almost certainly 

 concluded, intervenes somewhere ; but other transformations may 

 not improbably take place, . before the latter of these types is 

 reproduced. 



346. The Encysting process has been observed to take place 

 among several other forms of Infusoria ; so that, considering the 

 strong general resemblance in kind and degree of organization 

 which prevails throughout the group, it does not seem unlikely 

 that it may occur at some stage of the life of nearly all these 

 Animalcules, just as the 'still' condition alternates with the 

 ' motile' in the most active Protophytes (§§ 188-192). And it 

 is not improbably in the ' encysted ' condition that their dispersion 

 takes place ; since they have been found to endure desiccation in 

 this state, although in their ordinary condition of activity they 

 cannot be dried-up without loss of life. When this circumstance 

 is taken into account, in conjunction with the extraordinary 

 rapidity of multiplication of these Animalcules, and with the fact 

 that a succession of different forms may be presented by one and 

 the same being, the difficulty of accounting for the universality of 

 their diffusion, which has led some Naturalists to believe in their 

 ' spontaneous generation,' and others to regard them as isolated 

 particles of higher organisms set-free in their decomposition so as 

 to constitute an ' equivocal generation,' is as readily got-over as 

 we have seen it to be in the case of the Fungi (§ 268). Although 

 it may be stated as a general fact, that wherever decaying Organic 

 matter exists in a liquid state and is exposed to air and warmth, 

 it speedily becomes peopled with these minute inhabitants,* yet 

 it appears now to have been satisfactorily ascertained by the 

 carefully conducted experiments of M. Pasteur, that perfectly 

 free access of air to such infusions is essential to the appearance 

 of Animalcules as well as of Protophytes (§ 263) in them. For 

 having kept infusions of decaying Animal and Vegetable matter 

 in air which had been filtered (so to speak) of any floating germs 

 it might contain, by passing through a plug of cotton-wool, he 

 found that no Animalcules made their appearance under these 



* See bis very important " M£moire sur les Corpuscles Organises qui 

 existent dans 1' Atmosphere," in "Ann. des Sci. Nat." Ser. 4, Tom. xvi. 

 p. 5, which, disposes effectually of the doctrine of ' spontaneous genera- 

 tion.' 



