146 PHARMACEUTICAL BACTERIOLOGY 



there must be some elective affinity. That the elective affinity is only 

 slight is evident from the fact that the bacteria referred to will very readily 

 grow and multiply upon artificial culture media, and may be induced to 

 change hosts. Some algae show an elective affinity for certain living 

 substrata. Sirosiphon puhinatus occurs quite constantly upon species of 

 Umbilicaria and Gyrophora. , Pleurococcus punctiformis occurs upon the 

 young thallus of Cladonia and Baeomyces. Pleurococcus vulgaris, on the 

 other hand, occurs upon the most varied substrata living'and dead; hence 

 this association is evidently only accidental, as the alga shows no preference 

 for any particular host. It has, perhaps, a slight biologic preference for 

 some of the Polyporei. 



Some of the higher crustaceans select certain corals, among which they 

 live, without forming any marked symbiotic relationship. In one locahty 

 (geographical area) Hydra viridis seems to prefer one vegetable substra- 

 tum (Nuphar), while in another locailty it prefers to live upon another 

 plant, Lemna polyrhiza. Some Rotifera show a preference for certain 

 plants to which they attach themselves. Certain algae, as species of 

 Dactylococcus and Euglena, show a decided tendency to locate upon such 

 animals as cyclops, snails, and clams. Some mammals (sloth, ant-eater, 

 and others), have algae living upon them. The symbiosis of snails with 

 corals is perhaps contingent. Some sponges and hydroids show a prefer- 

 ence for animals, others for plants. Marine hfe in particular, presents 

 many forms of contingent symbioses. The instances cited are sufficient 

 to indicate the nature of contingent symbiosis. Many require further 

 careful study before anything definite can be stated as to their biological 

 activity and as to their relationship to other symbioses. 



II. Antagonistic Symbiosis 



.The phenomena included under this head are of wide occurrence and 

 were the first to receive the attention of scientists. The term as here used 

 includes mutual antagonistic symbiosis and antagonistic symbiosis proper.. 

 The former is not generally recognized by authors. The latter is more 

 commonly known as parasitism. There are no objections to the use of the 

 term parasitism, since it has become clearly defined and definitely restricted 

 in its application. It is, however, recommended that the term antagonis- 

 tic symbiosis be substituted for the sake of uniformity in terminology. 



From the nature of things the morpho-physiological specializations and 

 adaptations of antagonistic symbiosis are limited. Although one of the 

 symbionts may be highly benefitted the other is always injuriously affected. 

 This injurious effect may finally reach the stage where it will react upon 

 the parasite, .thus indirectly resulting in the mutual destruction of the 

 .symbionts. In far the greater number of instances the host is not de- 



