INFUSORIA. 835 



a number of similar globular nuclei, which multiply with mitosis in Opalina. 

 Irregularly shaped nuclear fragments are found as well in some species 

 of Holosticha ; and in the Holotrichans Choenia teres and Trachelocerca 

 Phoenicopterus the nucleus is reduced to chromatin granules scattered 

 irregularly, but the condition may not be a permanent one. The Opalinids 

 Benedenia and Opalinopsis^ which are endoparasitic in Cephalopoda, vary 

 at different times. The nucleus of the former is a long convoluted band, 

 which branches and segments into globular or irregular fragments ; that 

 of the latter is reticulate, but may break up into fragments. The Hetero- 

 trichan Plagiotoma Lmnbrici is said to resemble Benedenia. During fission 

 or gemmation the round or ovate nucleus and the paranuclei divide with 

 mitosis : the band-like nucleus is said not to do so. In many multinucleate 

 forms fission does not affect the nuclei 1 , but in Holosticha scutellum and 

 Opalinopsis their previous fusion has been observed. The small encysted 

 fragments of Opalina Ranarum^ O. obtrigona, and O. dimidiata become 

 uni-nucleate just before or just after their exclusion from the cyst. During 

 conjugation, the nucleus is resolved into fragments, the paranuclei divide 

 with mitosis. Interchange of a paranucleus, or of a portion of one, and 

 fusion of the interchanged structure, whichever it may be, with a cor- 

 responding one in situ has been observed (Gruber, Maupas). A new 

 nucleus and paranucleus are then formed from the old paranuclei. It is 

 not certain, however, how far the fragments of the old nucleus are excluded 

 from any share in the process ; whether they are extruded or absorbed 2 . 



The contractile vacuole is rarely absent, as in the Opalinid genera 

 Opalina^ Opalinopsis^ and Benedenia^ and in a few Hypotricha*. It is 

 nearly always superficial in position, and is usually single, but there may 

 be more, e.g. 2 in Paramecium, and Vorticellids with a well developed 

 cuticle, several in Chilodon, 50 in Trachelius ovum> or even 100 in Prorodon 

 margaritifer . Or small secondary vacuoles, irregular in position and appear- 

 ance, may be present, e. g. in Prorodon teres. When distended it is usually 

 globular, rarely linear as in some Opalinids. It is discharged by a pore, 



1 Maupas, op. cit. p. 654. 



2 Joseph has observed a natural fragmentation of the nucleus in Stylonychia, one induced by 

 confinement in the dark in Paramecium caudatum. An exchange of portions of the nucleus has 

 been seen by Schneider in Anoplophrya circulans, of portions of the paranucleus by Joseph, Gruber, 

 and Maupas. The first-named of the three has recorded movements of the chlorophyl bodies in 

 Ophrydium versatile indicating an interchange of protoplasm. Plate observed the resolution and re- 

 constitution of the nucleus in Lagenophrys ampulla when about to quit its tube, or after fission 

 (Z. W. Z. xliii. pp. 213-14), or during gemmation (Ibid. pp. 214-15). But in the permanent 

 conjugation of two young Spirochona gemmipara, he saw both nuclei and paranuclei fuse without 

 previous change (loc. cit. pp. 206-9). See the authorities quoted p. 840, post. 



3 See Maupas, A. Z. Expt. (2), i. p. 633. Maupas thinks (op. et p. citt.) that Strombidium 

 urceolare and S. sulcatum have also no vacuole. Saville Kent assigns one to them. So does Geza 

 Entz to Actinotricha, which has not got one according to Maupas, with the remark that there is a 

 long pause between the discharge and formation de novo of the vacuole. 



3 H 2 



