ai4
THE SEAS
the plant in _—the association is not always clear but it is
probable that it obtains valuable food material from the
,
waste products of the animal it lives in.
In the case of
the animal the advantage is more obvious; in -virtue of
its possession of the green colouring matter chlorophyll,
the plant, in the presence of sunlight, can manufacture
starch out of—the water and the carbonic acid gas it holds
.
in solution, and this starch‘ may be utilized by the animal.
,
The most striking case of this type of symbiosis is furnished by a small atworm, Conv01uta, only a few milli—
_
‘
-metres long, which lives on the sandy shores of north Brittany
.
and the Channel Islands.
It is bright in colour and occurs
'
in large colonies which form green patches on the yellow
sand, and is an animal of most
habits, suddenly
appearing from beneath the sand immediately after the
-
tide has left it-and disappearing just—— before the tide return's“
(Plate 82).
The green colour is due entirely to the presence
_
in the tissues of vast numbers of algæ.
These are not
'
present in the 6gg-but, the animals become infected in a very
early stage in development, and if they are kept free.from in—
fection by articial means they fail to d‘eVelop pr0perly and
.
,
soon die.
Although in early life it is able, like other-at—
worms, to feed on smaller animals the Convoluta soon nds
_
_
that it is much simpler to depend entirely on the starch
from the green plants and, as a result of disuse, its digestive
organs degenerate so that it cannot, even if it wishes,
feed like a normal animal;
This is the explanation of its
regular habits for in order to obtain the sunshine with0ut
..
which the plants cannot form their star‘ch, it has to expose
itself to the full glare “of the sun for aslong as possible,
,
that
exposed on the sand for the whole'time the tide
'
=
is out ; When thètide returns it. has to burrow or it would
be washed aWay.
'
.
But, owing probably to the fact that the Convoluta
_