196 



BIOLOGY OF THE PROTOZOA 



well assured at the present day, and the conditions in these para- 

 sites suggests the peculiar relation which Shibata (1902) has de- 

 described in the intracellular mycorhiza, where a mycelium thread 

 is grown straight toward the nourishing cell nucleus of the host, 

 causing marked hypertrophy- on the part of the cell. In Canju- 

 tropha, the nucleus of the host cell is pushed to one side and the para- 

 site assumes such a form that the nucleus lies in a small bay (Fig. 

 93, 2n). In the cytoplasm of the cell an intracellular canal is then 

 formed which runs from the host nucleus to the nucleus of the para- 

 site, and Siedlecki holds that the food of the parasite is all elab- 

 orated by the nucleus of the host cell, while the other spermatogonia 



Fig. 93. — Food-Getting adaptations of Sporozoa. 1, Pyxinia mobiuszi with epi- 

 merite deeply insuiik in the epithelial host cell (after Leger and Dubosq) ; 2, Caryo- 

 tropha mesnili with an intracellular canal from the nucleus of the host cell (ri) . (After 

 Siedlecki.) 



form a protective epithelial sheath around it. When the parasite 

 is full grown the cell is destnned and the bundle degenerates. 



It is difficult to draw the line between symbionts, commensals 

 and parasites. Symbionts are organisms living with a host in such 

 a relation that both are benefited; commensals are organisms which 

 live with a host without benefit or injury to the latter but to their 

 own advantage, and parasites are organisms which, to their own 

 benefit, cause injury in one form or other to the host. Symbiosis 

 is well illustrated by the harmonious life of some chlorophyll-bearing 

 forms, Zoochlorella, Zooxanthella, etc., and Protozoa in which the 

 former live {Paramecium bursaria, "yellow cells," Stentor viridis, 



