210
THE SEAS
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_
with their powerful teeth.
Seals and sea-lions
feed on sh which they pursue and capture in largenumbers,
.
but walruses live on sheilsh which they scrape up by means
of their_largetusks.
,
_
,
.
_
.
SUCKERS ÀND PARASITES
The only type of feeders which we have not yet considered are those which live by sucking in uids or soft
tissues.
These are not numerous in the sea except as
parasites, although & few animals such as some of the little
nudibranch molluscs or sea slugS which live On green weed
.are suckers without being parasitic, merely drawing in the
.
soft green
There are a
many parasites
_
_
which do not dWell within the b0dy of the animal on which
,
they“ are parasitic, but either x themselves‘permanently
For move about-on the outside Of the body.
These are
‘
known as ectoparasites
'
to
distingùish them from the
'-endoparasiteS' which live within the body of the'host.
_
T here, are many examples of ectoparasites in the marine
__
the commonest beingcopepod crustace‘àÿ ”relatives
_
of the freely swimming copepods which
in countless
.
numbers in the sea, forming, as we have seen, the principal
food of sh such as mackerel and hetring;
These‘are usually
on sh and’assume all manner of strange shapes;
.afewîof them can be recognized asCopepods butin the
_
'
majority it is onlÿ.‘a knoW1ed‘ge of their life—hist0rÿ inÿthe
early stages which has enabled us to classify_them as
copepods.
Examples of different types. of these parasites
‘
'
are given in Plate 81.
Some of the less degenerate kinds
move about freely on the Surface of their h05tsh, probably
_
living on the mucus and soft
but the more degeneraté
bne's bore
the, tissues of their host or x themselves
'
' _
_
'
to the gills, or” soft Skin found the eyes, and have usually