928 MINNESOTA BOTANICAL STUDIES. 



considered as of a symbiotic nature since the two generations 

 are parts of the same ontogeny. There is however no doubt 

 that the two generations form a highly specialized symbiosis 

 (individualism). 



There are many other phenomena of a complicated nature 

 which are designated as true parasitism by some authors while 

 others discuss them without referring them to any symbiotic 

 category. Some of these will receive mention in order to in- 

 dicate more clearly the complexity of the subject. 



Several species of crab belonging to the genus Stenorhynehus 

 are usually covered by a growth of algae, sponges and other 

 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 sig- 

 nificant 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 

 zoologic and botanic garden. Another crab is totally hidden 

 by sponges growing upon it which enables it to approach 

 its prey unpercieved as well as to hide it from its enemies. 

 Although some of these phenomena seem very complicated, 

 there is no evidence of marked symbiotism. If more than mere 

 accidental symbiotism does exist, no experiments 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 subterraneous attacks live snails, eats 

 the animal and then moves into the empty shell. The cray- 

 fish Phronima sedentaria eats species of Boliolum 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 a symbiotic nature, yet one must hesitate to 

 place them in this category, since the hunting, killing and 

 eating process is not true parasitism (antagonistic symbiosis). 

 According to definition, symbiosis necessitates a prolonged 

 contiguous relationship. This is not the case with the carniv- 

 orous animals and their prey. The apparently wonderful 

 adaptations of the crab and other animals to the snail-shell and 



