452 THE PROTOZOA 



adaptation to their mode of life (c/. Leger and Duboscq, 848). The best- 

 known genera are Anoplophrya, a typical ciliate with micronucleus and 

 macronucleus and with a rudimentary cytostome, constituting with Hopli- 

 tophrya, Herpetophrya Discophrya, etc., the group Anoplophryince ; Chromidina 

 and Opalinopsia, parasitic in Cephalopods, are probably allied to the fore- 

 going (cf. Dobell, 833). The species of Opalina, constituting the group 

 Opaiinince, are parasitic in frogs and various cold-blooded vertebrates ; 

 their nuclei vary in number in different species from two to an indefinitely 

 large number, but are all similar and without differentiation into micronuclei 

 and macronuclei at any period of the life-cycle. Cepede has monographed 

 the section Anoplophryince, and has described a number of new genera and 

 species, distributed amongst eleven families. The Astomata are internal 

 parasites of their hosts, especially of the digestive tract. Protophrya ovicola 

 occurs in the brood-sac of the mollusc Littorina rudis, and is parasitic upon 

 its eggs, causing their disintegration (Kofoid). 



The remarkable form Pycnothrix monocystoides, from the gut of Hyrax 

 capensis, described by Schubotz, deserves special mention. It reaches a length 

 of 3'2 millimetres, and contains parasitic nematodes. The animal itself has 

 a great superficial resemblance to a nematode or to a monocystid gregarihe ; 

 it has a very thick and distinct ectoplasm, covered by an oven coat of short 

 cilia, and with two longitudinal grooves which Schubotz regards as equivalent 

 to the peristomial grooves of other Ciliata. Each groove contains a series of 

 pouch-like depressions, which open down into the cndoplasm, and are provided 

 with special tracts of myonemes. Schubotz regards these pouches as a series 

 of cytos tomes, but no food- particles or vacuoles are found in the endoplasm ; 

 the interpretation, therefore, of these openings as cytostomes can only be 

 taken in a phylogenetic sense ; actually they appear to represent perforations 

 of the tough ectoplasm whickmay facilitate absorption of food by the osmotic 

 method. For the cilia myonemes and excretory system of this form see 

 above (pp. 443, 446 447) ; the micronucleus and macronucleus are each 

 single and of the ordinary type. Pycnothrix stands at present quite 

 isolated. 



The species of the genus Opalina differ in certain peculiarities of structure 

 and life-history from all other ciliates. The life-history of the common 

 species of Opalina parasitic in the rectum of the frog has been studied by 

 Metcalf (853) and Neresheimer (857), whose accounts agree as regards the 

 general life-cycle, but differ in some cytological details. 



Opalina ranarum multiplies in a vegetative manner during the summer 

 and autumn months, but in the spring a special propagative cycle occurs in 

 relation to change of hosts and is followed by sexual processes. 



The vegetative reproduction increases the numbers of the parasite in the 

 host ; it consists of two processes, multiplication of the nuclei and division 

 of the body, which go on independently. The animal contains a great many 

 nuclei, and when it reaches a certain size the body divides either longi- 

 tudinally or transversely to produce two daughter-individuals, each of which 

 grows again to the full size. The multiplication of the nuclei is effectedjay 

 a simple mitosis, similar to that of the micronucleus of other Infusoria, and 

 without centrosomes. 



In the spring the parasites divide rapidly and repeatedly, without growing 

 to full size between the divisions, so that they become continually smaller in 

 gize. A few individuals, however, do not undergo this process of rapid 

 fission, but remain of the ordinary type, forming a stock which persists and 

 carries on the infection in the frog, while those which divide up are destined 

 to pas,s out of it. At the beginning of the process of rapid division, the nuclei 

 extrude chromidia, some of which are absorbed, while from the remainder 

 secondary nuclei are formed (Neresheimer). Finally the old nuclei are 

 absorbed. The secondary nuclei also multiply by mitosis ; and, according 

 to Metcalf, in the later mitoses preceding encystment the number of chromo- 

 gotnea is reduced to one-half the ordinary number (in 0. intestinalis from eight 

 to lour, in O. caudata from six to three). The result of the repeated division 



