202 



BIOLOGY OF THE PROTOZOA 



sites suggest the peculiar relation which Shibata (1902) has 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 Caryotropha, 

 the nucleus of the host cell is pushed to one side and the parasite 

 assumes such a form that the nucleus lies in a small bay (Fig. 103, 

 2n). In the cytoplasm of the cell an intranuclear 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 

 form a protective epithelial sheath around it. When the parasite 

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



_ O. 



Fig. 104. — Ellobiophrya donacis, a peritrich with ring-form attaching organ which 

 passes around the gill bars of the lamellibranch. X 800 and 1350. (After Chatton 

 and Lwoff, Bull. biol. de la France et de la Belgique, 1929; courtesy of Prof. N. 

 Caullery and Les presses TJniversitaires de France.) 



Other special adaptations in the interest of food-getting are fre- 

 quently spectacular. Thus Ellobiophrya branchiorum (Chatton and 

 Lwoff, 1928), a commensal ciliate on the gills of the lamellibranch 

 Donax sp., has developed a curious, posterior, ring-form process 

 whereby it is firmly anchored to the gill bars (Fig. 104). 



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 



