INFUSORIA — ENCYSTING PROCESS. 421 



the name of Oxytricha. This possesses a long, narrow, flattened 

 body, furnished with cilia along the greater part of both margins, 

 and having also at its two extremities a set of larger and stronger 

 hair-like filaments ; and its mouth, which is an oblique slit on 

 the right hand side of its fore part, has a fringe of minute cilia 

 on each lip. Through this mouth, large particles are not unfre- 

 quently swallowed, which are seen lying in the midst of the gela- 

 tinous contents of the general cavity of the body, without any 

 surrounding " vacuole ;" and sometimes even an animalcule of 

 the same species, but in a different stage of its life, is seen in the 

 interior of one of these voracious little devourers (b). In this 

 phase of its existence, the Trichoda undergoes multiplication by 

 transverse fission, after the ordinary mode (c, d) ; and it is usually 

 one of the short-bodied "doubles" thus produced (b), that passes 

 into the next phase. This consists in the assumption of the 



f lobular form, and the almost entire loss of the locomotive appen- 

 ages (f) ; in the escape of successive portions of the granular sar- 

 code, so that "vacuoles" make their appearance (g); and in the 

 formation of a gelatinous envelope or cyst, which, at first soft, 

 afterwards acquires increased firmness (h). After remaining for 

 some time in this condition, the contents of the cyst become 

 clearly separated from their envelope ; and a space appears on 

 one side, in which ciliary movement can be distinguished (i). 

 This space gradually extends all round, and a further discharge 

 of granular matter takes place from the cyst, by which its form 

 becomes altered (k); and the distinction between the newly 

 formed body to which the cilia belong, and the efifete residue of 

 the old, becomes more and more apparent (l). The former in- 

 creases in size, whilst the latter diminishes ; and at last the former 

 makes its escape through an aperture in the wall of the cyst, a 

 part of the latter still remaining within its cavity (m). The body 

 thus discharged (n) does not differ much in appearance from that 

 of the Oxytricha before its encystment (f), though only of about 

 two-thirds its diameter ; but it soon developes itself (o, p, q) into 

 an Animalcule very different from that in which it originated. 

 First it becomes still smaller, by the discharge of a portion of its 

 substance ; numerous very stiff bristle-like organs are developed, 

 on which the animalcule creeps, as by legs, over solid surfaces ; 

 the external integument becomes more consolidated on its upper 

 surface, so as to become a kind of carapace ; and a mouth is 

 formed by the opening of a slit on one side, in front of which is 

 a single hair- like filament, which is made to turn round and 

 round with 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 be- 

 tween A and p, Q (Fig. 199), since the two last figures are drawn 

 under a magnifying power twice as great as that employed for 



