GENERAL _CHARACTERISTIGS
9
'
Stages. — Sometimes they are close together on the one side,
as in the asymmetrical at—shes (Fig. 3), or on theto_p
ofthe head, but as a rule theyàre separate, and the sh
be said only to have “
monocular
” Vision;
In the she‘s that
live in caves-or at greatïdepths in the ocean, the eyes:heive
degenerated, partly’ from Want of uSe, but moreespeciallÿ
because they require the rays of light
,
'
behind the retina, Without which they Cannot‘See; =
J
At the side of the head, behind the mOuth, isÏ the‘:làrge
and movable gill—cover (g, Fig. 1). ___On the inner side of this
'
,
lie,the gills in a large chamber below the hinder part of the
skull.… The “gin—cover is Supported by a number of bones and
moved by muscl€s
,
«
_
_
Which cause “it rt0 [
…
open and shut as the
,
_
water passes through: ,
_
the mouth and over'
”
,
_
the gills in breath,
'
'
-
_
'
«
'
ing. Sometimes the
_
'
external Opening is,
__
_
,
’
reduced .to a
,
small
'
slita’;
_
,
(Pipe—, ,…
—
Ÿ‘below,
-
Fm.) 5Ï.;_——'The Bat—sh(Màhhe
15e;
in L'sôineî’ ïEels}
t°,
°f î '
.
,
/Ü
"lives
water
about ’the‘- ‘_'W€St
the Sharks and, Rays
,(FfÔmC_et
a
',
there isno gill—'cover, -_ ,.
‘
*
' —
*Ï
-…;__"
and the gill—slitS or openings— can be seen externally just
'
behind the head.
—
,
f
_
,
head itself is the. most variable part of the sh;
naturally, for it comes under more varied inuences:_œ The
»
body moves—…it forwards or,swihgs it from sideto
,
>
,
the body' muscles,alsoact moredireetlyuporï the JaWS,
_
thus on the front part of the Skull. And
the head;
mOVCS ,_f : ,
it
Under the pressure of, the surrounding Water,wh1oh
has helped to
;theïshlapÿeaccordmgt@theraœOf ;
movement;
What
underthese };
Ï
inuences
__ may be.seenomseveral9ffhegures
;
,_.
,
i In