144 PROTOZOA 



In a few genera (Didinium, for instance) the course from mouth 

 to anus is a direct straight line, and one may almost speak of a 

 digestive tract. In Zoxodes and Trachelitis (Fig. 56) the endo- 

 sarc, as in the Flagellate Noctiluca (Fig. 48, p. 133), has a 

 central mass into which the food is taken, and which sends out 

 lobes, which branch as they approach and join the ectoplasm. 

 The endosarc contains excretory granules, probably calcium phos- 

 phate, droplets of oil or dissolved glycogen, proteid spherules, 

 paraglycogen grains, etc. 



The nuclear apparatus lies at the inner boundary of the 

 ectoplasm. The " meganucleus " may be ovoid, elongated, or 

 composed of two or more rounded lobes connected by slender 

 bridges (Stentor, Stylonychia), The " micronucleus " may be 

 single ; but even when the meganucleus is not lobed it may be 

 accompanied by more than one micronucleus, and when it is 

 lobed there is at least one micronucleus to each of its lobes. 1 

 The meganucleus often presents distinct granules of more deeply 

 staining material, varying with the state of nutrition ; these are 

 especially visible in the band-like meganuclei of the Peritrichaceae 

 (Figs. 51,60). At the approach of fission it is in many cases dis- 

 tinctly fibrillated. 2 But all other internal differentiation, as well 

 as any constriction, then disappears ; and the ovoid or rounded 

 figure becomes elongated and hour-glass shaped, and finally con- 

 stricts into two ovoid daughter-meganuclei, which, during and 

 after the fission of the cell, gradually assume the form charac- 

 teristic of the species. The micronuclei (each and all when they 

 are multiple) divide by modification of karyokinesis (or " mitosis ") 

 as a prelude to fission : in this process the chromatin is resolved 

 into threads which divide longitudinally, but the nuclear wall 



1 Gruber (Ber. Ges. Freib. 1888) has shown that in several marine Ciliata 

 the meganucleus is represented by an enormous number of minute granules dis- 

 seminated through the endosarc, which, on the approach of fission, unite into a 

 single meganucleus. As an adjacent micronucleus makes its appearance at this 

 stage, he infers that the micronucleus must be also resolved in the intermediate 

 life of the cell into granules too small for recognition under the highest magnifica- 

 tion attainable, and that they must then coalesce. 



2 In the peculiar Peritrichan Spirochona the division of the meganucleus is a 

 much more complex process than usual, and recalls that of the undifferentiated 

 nuclei of many Rhizopods (see Rompel in Z. wiss. Zool. lviii. 1894, p. 618). 

 Opalina has neither mouth nor anus, nor contractile vacuole, but a large number 

 of similar nuclei, that divide by a true mitotic process, like micronuclei. We 

 have referred it (pp. 114, 123) to the Flagellates, next to the Trichonymphidae. 





