INFUSORIA. 



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INFUSORIA. 



from which the cilia usually are retracted 

 or disappear ; a notch is first perceived, 

 which afterwards "becomes deeper, until the 

 body is completely cleft ; the two halves 

 then acquire cilia, and assume the functions 

 of perfect animals. In the transverse divi- 

 sion, a median constriction appears first, 

 followed by perfect separation, as in the 

 last. During these processes of division, 

 the animals sometimes continue their move- 

 ments as usual ; at others this is more or 

 less interfered with. In Vorticetta (PI. 32. 

 fig. 21 a), in which the process of longitu- 

 dinal division may be conveniently watched 

 on account of the comparative fixure of the 

 animals by a pedicel, when the division is 

 nearly completed a ring of cilia is formed 

 near the attached end of the body, by the 

 movements of which the new Vorticella is 

 separated from the parent. The process is 

 completed in about an hour. 



Claparede and Lachmann state that the 

 first process in the spontaneous division of 

 Vorticella is the development of a fresh 

 mouth-circle of cilia and a second contrac- 

 tile vesicle ; then a partial division occurs, 

 which is followed by fission of the nucleus 

 and final separation of the two animalcules. 



(b.) Fission as part of an encysting pro- 

 cess. Many of the Infusoria are observed to 

 alter their form at certain periods, become 

 rounded, lose or retract their cilia (PL 32. 

 fig. 27), and to secrete all over their surface 

 gelatinous matter, forming a coat or cyst 

 enclosing them. While thus encysted, the 

 substance of the body becomes divided, and 

 gives origin to a number of germs, w^hich 

 are discharged by the bursting of the cyst 

 (PI. 32. fig. 34). They do not then resemble 

 the parent, but are gradually developed, 

 during ordinary growth, into its form. 

 In some cases the progeny or brood become 

 individually encysted within the parent 

 cyst ; it appears, however, that they are not 

 discharged in this condition, but escape 

 first from their own cyst and then from 

 the parent, in which they leave their own 

 exuviae. Stein thinks that it was such 

 broods that Ehrenberg mistook for the re- 

 sults of the increase by diffluence, 



In Trichoda lynceus the encysting process 

 appears subservient to a kind of metamor- 

 phosis of the individual, the animalcule 

 which emerges from the cyst having cha- 

 racters in many respects different from the 

 Trichoda, but no multiplication is effected 

 either by subdivision or gemmation. The 

 late Jules Haime described this multi- 



plication by fission, the encysting of ova of 

 the separate Trichoda, and the subsequent 

 escape of a differently shaped creature, 

 which became gradually developed into a 

 form like Aspidisca. 



2. Gemmation is not a general proces in 

 the Infusoria. It is well seen in Vorticella 

 (PI. 32. fig. 26). The buds arise from near 

 the posterior end of the body, and, when 

 fully developed, liberate themselves by the 

 formation of a posterior ring of cilia, as 

 above mentioned. 



3. Conjugation. Balbiani supposed that 

 male and female organs existed in many 

 Infusoria, the former being represented by 

 the nucleolus, which resolved itself into sper- 

 matozoa ; the latter by the nucleus, which 

 after sexual conjugation became converted 

 into ova. Later researches, however, show 

 that the conjugation is an ordinary fusion 

 of the structures of two individuals without 

 sexual relations. In the conjugating con- 

 dition, the animalcules often closelyresemble 

 those undergoing division. Thus Parame- 

 ciuin aurelia multiplies to a great extent 

 by self-fission, but only to a definite extent, 

 for sooner or later they conjugate. They 

 assemble upon certain parts of the con- 

 taining vessel, and soon become coupled in 

 pairs. They are closely adherent to each 

 other, with their similar extremities turned 

 in the same direction and their mouths 

 closely applied. While thus conjugated, 

 they continue moving with agility in the 

 liquid, turning round and round upon their 

 axes ; but those which, like Stentor, are 

 attached by a footstalk remain almost 

 motionless. The conjugation lasts for five 

 or six days. It is also stated that reproduc- 

 tion is effected by the breaking up of the 

 nucleus into fragments which subsequently 

 become developed into the parental form. 



The metamorphosis of the Infusoria has 

 been noticed above ; but there is another 

 method by which the individual is pre- 

 served for a time, the encysting process 

 without fission. Many Infusoria at certain 

 times undergo an encysting process, which 

 apparently serves as a provision under cir- 

 cumstances which do not permit the con- 

 tinuance of their ordinary vital activity. 

 The movements of the Infusoria diminish 

 in vivacity, and the cilia are either lost or 

 retracted; the surface of the body pours 

 out a secretion which hardens around it 

 and the animalcule lives on and rotates 

 within its cyst until the time for its escape 

 arrives. 



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