22
_
, ”
THE SEAS
then a group of walking legs, and nally a series of swimming
legs under the tail region.
Allied to them are the
cally mysterious sea spiders or PYCNOGONIDA, resemblig
the true spiders in little beyond the possession
et four pairs
'
_
oilong legs.»
"
,
The MQLLUSCA form another great group.
The Gastropoda or univalved shellsh, such as the limpets, periwinkles
andï snails, the great majority of which are marine (the '
chief exceptions are snails and slugs), have usually a shell;
always of one piece, though the land and sea slugs have either
-*
no
shell or else one greay reduced and covered, over
With skin… Most of them live on the shore or on the sea
_
'
bottom but a few, without, or With greatly-reduced shells,
"
swim about near the surface, the sea butteries being the
'
commonest.
The
bivalve
_
molluscs
or
Lamellibranchia
have a shell composed of two equal, or almost equal, ha'lvss
,
united by an elastic ligament which forces the two halves
open except when they are drawn together by the powerful
adductor muscles.
They are all either marine or freshWatef animals, well—known examples being the mussel
and the ‘oyster.
third great division of the Mollusca
_
includes the Octopus, squid and cuttlesh, and is called the
Çephalopoda. Its members are all marine and are very
highly "organized animals with a head having a pair of
'
complex eyes and surrounded with a series of arms possessing suckers and hooks.
Some, such as the octopus, have
no shell ;. others,thecuttlesh is an example, have a broad
-“ bone ”« down the back which is covered with esh, While
:
a” few, like the pearlÿ Nautilus, have a large shell in which
,
.,
.
,
,
-
Î
Reæmbling the bi6alves in the posseSsion of a shell
.
consisting of two valvesare the BRACHIOPODA, usually only
found in deep waters-in our own seas.
The two halves of
the shell however, are always dissimilar, while the internal