152 PHARMACEUTICAL BACTERIOLOGY 



tropics, which has already been referred to. A given species of ant lives 

 upon and obtains its food supply from the branches of a tree (Cecropia) ; 

 in return the ants protect the tree against the attacks of another species 

 of ants. The ants live within the transversely divided hollow stem tO' 

 which they gain access by eating away the thin lateral (outer) area. 

 The thin outer membrane of which there is one to each hollow chamber 

 and the chambers themselves are, however, perhaps not the result of the 

 symbiotic association. The preexisting morphological characters simply 

 happen to make possible the establishment of this particular forni of 

 symbiosis. 



In the insectivorous plants (Drosera, Dionaea, Nepenthes) we doubtless 

 have another example of mutuahsm. Formerly it was generally believed 

 that the plant itself digested the insects which it caught, by the aid of 

 irritable glandular hairs and other special organs. It is highly probable 

 that the insect digesting ferment is secreted by bacteria which live upon 

 the plant. 



A most remarkable instance of mutualism occurs in the animal king- 

 dom. The very inactive polyp Actinia prehensa lives firmly attached to 

 the inner sides of the claws of the crustacean Melia tessellata. The 

 Actinia aids in kiUing the prey of the crayfish while the latter carries its 

 guest from place to place thus giving it better opportunities for securing a 

 sufficient food-supply. Mobius states that this association occurs with 

 all the representatives of Melia tessellata, both male and female, and 

 that it is almost impossible to remove the symbionts without injuring 

 them. 



Many of the symbiotic associations of algae with animals are perhaps 

 mutuaUstic. Many Actinias contain single-celled alg£e which elaborate 

 food-substances for the use of the polyp. Brandt states that as long as 

 this animal contains no algae, it feeds upon the organic substances in the 

 immediate vicinity, but as soon as it becomes associated with the alg£e it 

 depends upon these for the supply of organic food. Further research is 

 necessary to determine whether or not this is true mutualism. 



3. Individualism. — This form of symbiosis differs from mutualism in 

 that one or more of the symbionts is absolutely dependent upon the other 

 for its existence. It therefore represents a more highly specialized form 

 of mutuahsm, from which it is no doubt phylogenetically derived. 

 IndividuaHsm may be divided into semi-individuaUsm and complete 

 individualism. In the former at least one of the symbionts is incapable of 

 existing independently, however, the organism of which it was a part can- 

 not survive. In complete individualism none of the several symbionts 

 can continue to exist independently. A new individual, a new autonomy, 

 is the result. It cannot be denied that the association of the symbionts 



