198 THE ANIMAL PARASITES OF MAN 



Class IV. INFUSORIA, Ledermuller, 1763. 



The Infusoria (or Heterokaryota, Hickson, or Ciliophora, Doflein) 

 include the Ciliata and the Suctoria. A few authorities, including 

 Braun, raise the Suctoria (or Acinetaria) to separate rank as a class, 

 but this is not widely followed. 



The body of the Ciliata usually is bilaterally symmetrical and is 

 enveloped in a cuticle which has numerous openings for the 

 protrusion of the cilia. Most kinds have a fixed shape, whilst 

 changes in the form of others are brought about by the contractions 

 of the body substance. The latter exhibits hyaline ectoplasm, in 

 which myonemes, and occasionally also trichocysts (minute spindle- 

 shaped bodies) appear, and granular endoplasm which may contain 

 numerous vacuoles. The cilia, on whose various arrangements 

 the classification is based, are always processes of the ectoplasm. 

 Their form varies ; they may be hair-like, or more rarely thorn-like, 

 spur-like, or hook-shaped ; undulatory membranes also may occur, 

 which are probably composed of fused cilia. 



With the exception of some of the parasitic species, an oral cavity, 

 peristome or cytostome, is always present. It is frequently beset 

 with cilia or provided with undulatory membranes, which help to 

 waft the food inwards; sometimes there is an anal aperture (cytopyge) 

 generally placed at the opposite pole of the organism. A cyto- 

 pharynx fringed with cilia or sometimes with a specialized supporting 

 apparatus is connected with the peristome. Vacuoles form round 

 the ingested food, and in many species a constant rotation goes on 

 in the endoplasm. Often one, and sometimes two contractile vacuoles 

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

 surrounding temperature. Sometimes special conducting channels 

 lead to the vacuoles, or there are outlet channels leading to the exterior. 

 There is in almost every case a large nucleus (macronucleus), 

 and lying close up to it a small nucleus (micronucleus). The form 

 of the large nucleus varies according to the species. Numerous 

 nuclei are not very common, but these occur in Opalina, which lives 

 in the hind-gut of amphibia, 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 repeated many times, but finally cease, and then the conjugation 

 of two specimens brings about a regeneration, particularly of the 

 nuclei. Numerous examinations (Biitschli, Hertwig, Maupas, Calkins) 

 have demonstrated that after two individuals have associated by 

 homologous parts of the body, the micronucleus separates from 

 the macronucleus, becomes larger and divides twice by mitosis, so- 



