146 THE PROTOZOA 



B. The Nucleus 



With the exception of the multinucleate Myxosporidiida, the Sporo- 

 zoa are mononucleate. Schneider ('81) abandoned the attempt to 

 compare the nuclei in Sporozoa with those of ordinary animal and 

 plant cells, because of their peculiar structure. In most cases they 

 consist of a firm arid resisting membrane containing a single large 

 chromatin reservoir or karyosome, and are apparently without a linin 

 reticulum, such as is found in the nuclei of Metazoa. In some forms 

 the nucleus is similar to that of the Sarcodina and Mastigophora, 

 consisting of membrane, reticulum, and one or more chromatin 

 reservoirs. Recent observers have found that the different appear- 

 ances of the nucleus are characteristic of different stages of nuclear 

 activity, and that the reticulum, and even the nuclear membrane, are 

 derived from the karyosome, which in the sporozoite appears as a solid 

 homogeneous sphere of chromatin. 1 In the active phases the nuclei 

 of the Sporozoa differ widely from those in other Protozoa, the most 

 striking point of difference being the disappearance of the nuclear 

 membrane during division. The chromatin reservoirs may divide 

 directly, thus simulating the entire nucleus, or they may break down 

 into small chromatin granules, resembling the first stages of chromo- 

 some-formation in the flagellate Noctiluca. In the former there is no 

 distinct spindle, in the latter the completed spindle-figure has two sets 

 of fibres, although, according to Wolters's ('91) description, the fibres 

 seem to have a different function from those in the mitotic figures of 

 the Metazoa, since there is no connection between them and the 

 chromatin. 2 



C. Food-taking 



Like all endoparasites, the Sporozoa absorb fluid food through the 

 body-wall, even when, as in the Myxosporidiida, pseudopodia are 

 present. There is probably no specialized area devoted to food- 

 taking, but all parts are equally receptive. It is believed that in 

 some cases, notably in the Gregarinida and Myxosporidiida, minute 

 pores perforate the membrane between the outer markings. Although 

 the taking of food has never been observed, the indirect effects are 

 seen in the rapid growth of the parasite when in a suitable medium. 

 Thus a young gregarine, when it penetrates an epithelial cell (Fig. 

 77, A), is a minute ball of protoplasm; but it rapidly grows until it 

 occupies the greater part of its host, often forcing the nucleus to one 

 side. As it continues to grow, the front wall of the cell is pushed 

 outward until it finally breaks, and the lower portion of the parasite 



1 Cf. infra, p. 253. 2 See infra, p. 259. 



