SYMBIOLOGY THE BIOLOGICAL RELATIONSHIPS OF ORGANISMS 141 



same ontogeny. The two generations form a highly specialized symbio- 

 sis (individualism). There are many other phenomena of a compli- 

 cated nature which are designated as true parasitism by some authors 

 while others do not ascribe to them any symbiotic relationship. 



Several species of crab belonging to the genus Stenorhynchus are usually 

 covered by a growth of algae, sponges and other lower plants and animals. 

 This is perhaps a case of accidental symbiosis. The habitat of the crab 

 combined with its slow movement makes the chitinous skeleton a suitable 

 substratum for the attachment of various aquatic organisms. The cover- 

 ing may serve some protection but this is evidently of no significant 

 importance. Species of the closely related genus Inachus are also covered 

 by a similar growth but here the plants and animals serve as food for the 

 crab. Brehm states that the crab even transplants hydroids, algae and 

 other organisms upon its back, thus converting itself into a traveling 

 economic zoologic and botanic garden. Another crab is totally hidden 

 by sponges growing upon it which enables it to approach its prey un- 

 perceived as well as to hide from its enemies. Although some of these 

 phenomena seem very complicated, there is no evidence of marked sym- 

 biotism. If more than mere accidental symbiotism does exist, no experi- 

 ments have been made to demonstrate whether it is antagonistic or 

 mutualistic. 



The hermit crab is morphologically adapted to live in the empty shells 

 of certain snails. The last pair of legs are much shortened and serve the 

 special function of holding the shell. The coleopter Necrophilus subter- 

 raneous attacks live snails, eats the animal and then moves into the empty 

 shell. The crayfish Phronima sedentaria eats species of Doliolum and 

 Pyrosoma and utilizes the empty skeleton as a dwelling place, paddling 

 it about by means of its claws. Although these phenomena are in part of 

 symbiotic nature, yet one must hesitate to place them in this category 

 since the hunting, killing and eating process is not true parasitism) antago- 

 nistic symbiosis). According to definition, symbiosis necessitates a pro- 

 longed contiguous relationship. This is not the case with the carnivorous 

 animals and their prey. The apparently wonderful adaptations of the 

 crab and other related animals, to the snail-shell and to the outer skeletons 

 of crustaceans, is perhaps purely accidental unless it can be proven that 

 the structural conformations are the result of phylogenetic development. 



Climbing plants are interesting as they mark the beginnings of a highly 

 complicated form of symbiosis. The plants form a close association with 

 their supports, which in most cases are living plants; especially is this the 

 case in the dense jungles of the tropics. Whether these plants cling to 

 their support by means of twining stems, tendrils, suctorial organis or 

 aerial roots, there is more or less absorption of soluble food-substances 



