46
_
THE SEAS
Another is furnished by a tropical crab (llÆelia tessellata),
.
which carries anemones in each of its claws, rst deliberately
removing them from the rocks.
It
holds
them
fully
expanded, pushing them forward when attacked and taking
out the food theycapture and transferring it to its own
'
Ï
mouth.
The claws are used exclusively for carrying the '
anemones.
advantage to the arab is obvious while the
anèmone, in spite of losing so much of its food—, perhaps
gains, as by being.moved about in this manner it
so
,
many more opportunities of obtaining food.
'
‘
ADAPTATIONS FOR BREATHING
_
,
The problem of respiration—of obtaining that essential
.
.
minimum of oxygen, without which-no animal can live——
is often serious on the shore, where the inhabitants are part
of the time in water and the remainder in air.
We have
.
not Space
here to discuss the different organs, or gills, used
'
_
in differentanimals for obtaining oxygen, but some instances
of shore animals Which are able to respire both in air and
_
in water will be of interest.
This can be done, in a sense, by
[
a variety of animals which are able to keep their gill_s moist
7
for considerable periods while out of water, for example, the
Crustacea, which have gills covered over by the edge of the
'
shell—as in the higher forms like the lobster— and crabs—or
iii-the form of plates beneath the body. Of the different
,
periwinkles, those which live nearest the shore and are
often out of water for long periods, have developed the
j
power of breathing air, the walls of the gill'cavitÿ having
become
laden with _ne blood
whereby the
…blood is able to take up its oxygen from the air.
There are
crabs which live exclusively on land, soma, such as the
_
tropical rohber crab, only going to the sea to breed, and
[
having developed a true lung for breathing, while others
which have retained their gills have
”go downto the sea