PARASITES OF PROTOZOA 1025 



but have come to occupy a normal place in the metabohsm of the 

 combination. Other bacteria occur as occasional endobionts, present 

 in a variable percentage of hosts; often there are certain types that are 

 more or less likely to occur. Some of these are not noticeably detri- 

 mental to the maintenance by the host of normal activity. At the other 

 extreme are some that cause fatal diseases; this is true especially of the 

 nuclear parasites. 



ASSOCIATIONS OF A CONSTANT CHARACTER 



Instances in which endobiotic bacteria are always or at least usually 

 present are known chiefly in flagellates of termites and amoebae of the 

 genus Pelomyxa. Doubtless there are many other such associations 

 among Protozoa. Miyashita (1933) found the occurrence of abundant 

 rod-like bacteria in the cytoplasm of Ptychostomum (Hysterocineta) 

 bacteriophilum to be characteristic of the ciliate; and similar rods were 

 seen by Studitsky (1932) in the endoplasm of P. chattoni. Flexuous 

 rods from 8 to 20 p long were observed by Chatton and Lwoff (1929) 

 in all specimens of Ellobiophrya donach, but not in the mantle cavity of 

 the lamellibranch host of the peritrich. Furthermore, the rods often 

 showed division and were never corroded; so that these authors con- 

 cluded that the microorganism is a specific symbiont. 



Schizomycetes in Pelomyxa. — So characteristic are bacteria in Pelomyxa 

 that Penard (1902) designated the genus as always provided with 

 symbiotic bacteria. Greeff (1874), who observed them in Pelomyxa 

 paliistris, considered them to be crystals, and he had at first held them 

 to be seminal threads. F. E. Schulze (1875), though noting their 

 similarity to bacteria, agreed with Greeff that the rods are peculiar 

 structures of the Pelomyxa body. Leidy (1879) observed rods in his 

 Pelomyxa villosa (which species, according to Penard [1902], repre- 

 sents nothing in reality, the name having been applied to an aggregate 

 of several species of Pelomyxa), and noticed, as had Greeff, that many 

 appeared to be transversely striated. Bourne (1891) identified the rods 

 as bacteria, and Penard (1893) expressed the same opinion as to their 

 nature. 



Penard (1902) described the bacteria as having a length of from 

 10 to 15 |j, or sometimes 20 j_i; and in one individual there were rods 

 of from 40 to 50 p. Leiner (1924) found the rods varying in length 



