GENERAL CHARACTERISTICS
15
The
likewise products of the skin ’s activity,
and are formed in -tWÇ) diîerent ways. In the Sharks and
Rays the skin is, as it were“; blown out into minuté papillæ
or cones.
The outer layer or epidermis of the cone becomes
hard likeenamel by the deposition of chalk, Whilst the dermal
part inside forms dentine’with a central pulp cavity con—
taining the“ blood—vesSels_ and nerve. These give rise
rough ‘denticles or placoid scales, which. are
to
.
meet With on
or Dog-sh. . The teeth are fotde in
asinlaiWay. , T—he base of the_cone spreads out in the shape
leaving a hole in the centre:£0f the blood—
vessels1:
».
—Ï
"
_
,
,
A few of the bony shes have similar denticles, and these
may—also be presentin the;la—rval stages, but as a rule scales—
_
the inner layer of the”
-
…
;
proti‘uding denticle:or spine
earliest
(D1pn01)
,
had scales of asimilaf kind; ? As the ,plaÏe,'gfoivs, it mayfform
_
ifoughly_ foù1:—sidedi
injco--*eachf
'
shesof thePïeisent-daïy it{iS” Circular
'
…
‘
oroval(cyclœcl80ale)
;
:
.
j
«
rhombicland thefcycîoîd?
underlyingmusclesandthecontractwnsofthel«ttæhave
……11y à œ-dency tk1thknntmbd
.
areas. '
A
_
change in the
for ' '
example, il the
altei‘ the -shàpeo£
theseareas. _Inÿgÿhiswayit is supposéd, the rhombic
-
to. the: Cyc10id scales,
,
-
of 1noÿement;.;jAtfthe same
fthdntptfnlwldldtthl
'
Obliiqùë1ÿ,ïêah‘ëäthusi;
thehmder
-
©0veung0fthescalwverynnandthesealeseemîobe
-.
'
1Ylng0nïesurfaœ
_
»