1 88 THE PROTOZOA. 



dorsally in the middle of the body, but is connected with the terminal 

 aperture by a long canal. In the Peritrichida there are one or two 

 vesicles, which empty by contraction into special reservoirs, and these, 

 in turn, empty into the oesophagus (Fig. 102). 



According to Delage ('96) the contraction of the vesicle is brought 

 about by the contractility of the surrounding cortical plasm. Rhum- 

 bler ('98) has shown, however, that the contained water may so affect 

 the plasm that it becomes differentiated like ectoplasm, and this gives 

 some substantial basis for the view that a special membrane surrounds 

 the vacuole. There is no evidence, however, to show that this modi- 

 fied protoplasm acts like a sphincter. Biitschli holds that the vesicle 

 contracts through a mechanical force exerted upon the thin plasmic 

 layer between the vesicle and the opening of the excretory pore by 

 the pressure of the filling vacuole and the turgor of the cell. At the 

 completion of the diastole the pressure becomes too great for the 

 lamella, and the latter is ruptured, allowing the contained fluid to pass 

 to the exterior. 



C. THE NUCLEUS 



The nuclei of the Infusoria show some of the most striking 

 structural characteristics connected with the Protozoa. Here there 

 is a differentiation of the nuclear material into two forms, a larger 

 macronucleus^ and a very much smaller micronucleus. With the single 

 exception of Polykrikos (Dinoflagellidia), this differentiation of the 

 nuclei is found nowhere outside of the present group. The functions 

 of the two kinds of nuclei are supposed to be respectively vegetative 

 and reproductive (Biitschli), but this distinction is, perhaps, too 

 sweeping. Julin ('93) held that the macronucleus stands not only 

 for nutrition, movement, sensation, and regeneration, but for asexual 

 division as well, in fact is a " somatic nucleus," while the micronucleus 

 functions only as a sexual nucleus. There may be one or many of 

 each kind in each cell. The macronucleus, which is invariably 

 present, recalls the nucleus of tissue cells. It is usually single, and, 

 lying in the endoplasm, it may be carried about with the flowing 

 granules, or maintained in a permanent position in the cortical plasm, 

 or by processes from this plasm {e.g. Isotricha). Its form is quite 

 variable and has little significance for systematic work, for in the same 

 species under certain conditions it may even become amoeboid (Loeb & 

 Hardesty, '95). The usual form is spherical, but it may be elongated 

 into an oval, or into a flattened rod which may be curved or straight, 

 or it may be divided into small pieces resembling a string of beads, 

 connected by a membrane (Fig. 103). It is always provided with a 

 membrane (Maupas), but the chromatin contained within it is vari- 



