INFUSORIA 



hair-like, or more ' rarely thorn-like, spur-like, or hook-shaped ; 

 undulating membranes also occasionally occur. 



With the exception of some of the parasitical species, an oral 

 cavity, PERISTOME or CYTOSTOME, is always present ; it is fre- 

 quently beset with longish cilia or provided with undulating mem- 

 branes, which help to engulf the food ; frequently also there is an 

 anal aperture (cytopyge) generally placed at the opposite pole. A 

 CYTOPHARYNX clothed with cilia or supported by ligaments is joined 

 on to the peristome. Vacuoles form round the ingested food, 

 and in many species a constant rotation goes on in the endo- 

 sarc. Frequently one, sometimes two, contractile vacuoles are 

 present, the frequency of the pulsations of which depends on the 

 surrounding temperature. Sometimes there are in the plasma special 

 conducting channels to the vacuoles or outlet channels leading to 

 the exterior. 



There is almost in every case a LARGE NUCLEUS (macronucleus), 

 and lying close up to it a SMALL NUCLEUS (micronucleus) ; the form 

 of the large nucleus varies exceedingly according to the species. 

 Numerous nuclei are of rare occurrence (as in Opalina, which 

 lives in the terminal gut of amphibians, and is also distinguished 

 by the absence of an oral aperture). 



Reproduction is effected by binary fission ; less commonly, after 

 encystment, by multiple division, or by budding. The divisions 

 can be jnany times repeated, but finally cease, and then the con- 

 jugation of two specimens brings about a regeneration, particularly 

 of the nuclei. Numerous examinations (Butschli, Hertwig, Maupas) 

 have demonstrated the fact that after two individuals have joined 

 by homologous parts of the % body, the micronucleus separates from 

 the macronucleus, becomes larger and divides twice by the karyo- 

 kinetic process, so that four micronuclei are present in each one of 

 the two individuals forming the couple (gamete). Three of these 

 nuclei perish and become absorbed, the fourth gradually shifts to 

 the connecting bridge of the protoplasm, which has originated by 

 absorption of the cuticle at the point of contact of the gametes ; 

 and changes half of its substance for half of the substance of the 

 fourth micronucleus of the other gamete ; a new nuclear body is 

 thus formed and divides, usually twice, in each gamete. Of the 

 segments thus originated one becomes a micronucleus, and one 

 or several of the others, as the case may be, form or amalgamate 

 into a new macronucleus, the old macronucleus usually perishes 

 or becomes absorbed towards the termination of the conjugation. 

 Usually, sooner or later, the two gametes separate and again increase 



