THE PELAGIC FAUNA AND FLORA. 



215 



the radiolai'ians and Velellae (Fig. 138), in which the provision 

 of philozoon is most abundant, survive longer in confinement 

 than other pelagic animals. The common Polyclo- 

 nia of Florida can be kept for weeks in jars, un- 

 doubtedly owing to the mass of philozoon found 

 in its oral arms. 



Similar associations which do not have the same 

 physiological value are found throughout the an- 

 imal kingdom. It is difficult to draw the line be- 

 leiia Medusa twecu functional symbiosis and mere associations of 

 with yellow position, in such cases as that of the Vorticellidse, 

 which fix themselves anywhere ; that of the Bry- 

 ozoa, growing on fronds of sea-weeds, or the tests of crabs or of 

 moUusks ; or that of many kinds of 

 algse and sponges, found growing on 

 the carapace of Crustacea. A similar 

 association is that of the hermit crab 

 and its house, of Phronima and its Do- 

 liolum quarters (Fig. 139), neither of I 

 which differs greatly from the some- C 

 what less passive relationship of the ^ 

 oyster and its crab ; of Fierasfer and the 

 holothurians ; of ophiurans and crusta- 

 ceans living in the body of sponges ; 

 or of fishes swimming about in the 

 midst of the tentacles of medusse. 



More intimate still is the relationship existing between some 

 species of crabs living in the branches of coral, or of worms 

 or mollusks covered by gorgoniae and crabs, where the presence 

 of an associate modifies considerably the structure and mode 

 of growth of the abode. From this we pass gradually to 

 parasitism proper, where the organs of nutrition are specially 

 developed, and those of locomotion degenerate. Again, free 

 parasitism, we might almost call it, such as exists between in- 

 sects and plants, is readily traced in a sort of commensalism of 

 animals belonging to different classes of the animal kingdom, 

 living together in a relationship almost close enough to be con- 

 sidered as functional ; such as exists, for instance, between an- 



Fig. 13i;). — Plironima seden- 

 taria. ?. 



