PARASITISM IN PLANTS AND ANIMALS. 289 



pressed there for about ten minutes, at the end of which time it 

 drew away first one claw and then the other ; and beginning to 

 walk away it was seen that the Adamsia was once more fairly 

 adhering and in the right place. 



It is evident, then, from these observations, that the hermit- 

 crab appreciates the companionship of the anemone and doubt- 

 less finds it in some way conducive to his health or safety, nor 

 can we reasonably doubt that the advantage must be reciprocal, 

 though in what way it is not very easy to see. 



The tenancy of the living mussel-shell by a little pea-crab has 

 been known since the time of Pliny, who supposed the crab to 

 act as a sentry, warning the mussel of approaching danger, and 

 by a tweak of his claws causing him to shut up. Deep-sea 

 sponges are often found completely riddled by colonies of intri- 

 cately branched polyps, and one genus of sponges {Hyalonema), 

 known as the glass-rope sponge, is constantly so encrusted ex- 

 ternally by a polyp, that until very recently the sponge itself 

 was looked upon only as an adventitious growth. 



Lastly, let me mention very briefly an interesting case of 

 the presence of vegetable parasites — or perhaps commensals — 

 in minute animals of the very lowest grade. There is a very 

 numerous family of minute, often microscopic, floating marine 

 animals, each of which consists usually of a spherical or radiated 

 and beautifully perforated central shell, the shell being filled 

 with and surrounded by a soft gelatinous flesh. These belong 

 to the Class Radiolaria. In the soft parts may almost always 

 be seen numerous little yellow spheres, which have been shown 

 to be of a vegetable nature and are looked upon as parasitic algae ; 

 and the way in which plant and animal are mutually benefited 

 by the companionship is thus explained by Professor Geddes : — 



After confirming Haeckel's discovery of the presence of starch 

 and the observations of others on the survival of the yellow cells 

 after the death of the radiolarian, Geddes demonstrated the truly 

 algal nature of these cells from their cellulose walls, the identity 

 of their yellow colouring matter with that of diatoms, and the 

 evolution of oxygen under the influence of sunlight. It was 

 pointed out that the animal matter covering these cells must 



