FEEDING OF MARINE ANIMALS
215
needs a more varied diet than that supplied by starch, it
.
begins after a time” to feed upon the algèe—to kill the geese
which laid the golden eggs_—so that they gradually disappear,
their owner at this stage presenting the strange appearance
Of an animal with a green head and a white tail.
Finally,
,
having killed its best friends, the little atworm dies of
starvation, though not before it has laid large numbers
of eggs for the maintenance of
race.
This may be
considered a case of symbiosis'which has gone too far,_for, -
although the algæ can live freely in the sea and they are
by no means dependent upon the atworms, the latter,
after originally no doubt sheltering
algæ in return
_
for surplus food, have nally become entirely dependent,
essentially parasitic, upon them, and cannot even develop
if they are absent.
.
.
.
In the Giant Çlams and their allies (Tridaçnidæ) the
animals farm zooxanthellæ in the _greatly thickenèd tissues
which project between the open shell valves.
In,this case
'
the
the power of feeding on minute plankton,
'
_
like all -bivalve molluscs, and, although the contained
algæ form a valuable subsidiary source
the animal
-
never-becomes completely dependent on the plants, and,—
in—consequence, never f‘suers premature
Con—
_
voluta.
In “the coralsthe zooxanthellæ, as
noted»
inChapter VII, are never digested but they certainly act
'
as automatic organs of excretion.
The possible signicance
'0f: this has alreadybeen discussed. ,
-
—
,
'
.