THE PELAGIC FAUNA AND FLORA. 215 
the radiolarians and Velellze (Fig. 138), in which the provision 
of philozoón 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 philozoón found 
in its oral arms. 
Similar associations which do not have the same 
physiological value are found throughout the an- 
ju us E imal kingdom. It is difficult to draw the line þe- 
lella Medusa tween functional symbiosis and mere associations of 
with yellow position, in such cases as that of the Vorticellidæ, 
cells, magnified. cee 
which fix themselves anywhere; that of the Bry- 
ozoa, growing on fronds of sea-weeds, or the tests of crabs or of 
mollusks ; or that of many kinds of 
alow 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 
which differs greatly from the some- 
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 fabis swimming about in the Fig. 139.— Phronima. seden- 
midst of the жиы of medus:. ќагіа. 1 
More intimate still is the relationship existing between some 
species of crabs living in the branches of porn, or of worms 
or mollusks covered by gorgonie 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 сао 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- 
