the parts, in the case of the eel, but in
the other fishes which I have examined the
surface of the scale is very partially covered
by the enderon, being in its centre, at any rate,
in'contact with the cellular ecderon.
The vascularity of the scale never extends
to its most superficial layers, and may be ex-
plained in the same way as that of the test of
an Ascidian, which however is unquestionably
an ecderonic structure. The passage of its deep
layers directly into the connective bundles of the
enderon, which Ley dig has observed in Poly-
pterus (and which I will not sayjdoes not occur
elsewhere, though I have not observed it),
would appear to me only to indicate that this
scale, and perhaps others, are composed of two
portions, a superficial ecderonic part extend-
ing as far as the most superficial vascular
canals, and a deep portion beneath these be-
longing to the enderon.
However, all these points can only be de-
cided by a much more extensive series of in-
vestigations, principally directed to the ascer-
tainment of the position of the protomorphic
line and of the direction of growth of the
constituents of every scale, than I have hither-
to had time or opportunity to carry out ;
and as the attention of other observers does
* On the Structure and Development of the
Teeth, Quarterly Journal of Micros. Science, 1852.
Sup p.
not appear to have been directed to these par-
ticular points, the question must for the
present remain undecided.
Professor Williamson in his valuable and
philosophical contributions to our knowledge
of this subject (Phil. Trans. 1849-1852)
laid the foundation for a comprehension of
the mode of development of fish-scales, by
pointing out that Agassiz's views, though
essentially true, yet require a certain modifi-
cation. For though a fish-scale does really
grow by the apposition of layers to its deep
surface, as Agassiz asserted, yet it is not in-
cluded in a sac of the epidermis (if by that
term we are to understand the ordinary
cellular ecderon) ; and it is also true that its
deeper portions grow by their superficial
surface. Professor Williamson points out, in
fact, that every fish-scale consists of at least
two portions, a superficial homogeneous, or
at most canaliculated, laminated layer, the
ganoin (so called enamel or horny layer of
authors), and a deeper, also laminated, fre-
quently fibrous or osseous portion commonly
traversed by Haversian canals. Now these
two portions have a certain independence in
their mode of growth, at any rate after their
first formation, as may be easily understood by
the accompanying diagram {fig. 307.), which
represents a series of imaginary sections of
scales from their first growth onwards ; a, is
the protomorphic plane; 6, 6", the deep ecde-
ron ; 6', the superficial cellular ecderon, and the
line x, the centre of the scales from which
development commenced.
Kg. 307.
Suppose A to be the youngest scale, con-
stituted merely by a thickening and calcifica-
tion of the deep ecderon, which in u has
added several layers by apposition to its inner
surface, all of which retain the ganoin struc-
ture except the deepest, which becomes fibrous
in its texture, and forms the commencement
of the " Lepidine" layers of the scale ; these
layers, however, being as much a part of the
ecderon as the former. In c the scale widen-
ing, the edges of its " Lepidine " layer do not
remain in contact with the ganoin layer; but it
will be obvious that the re-entering angle
thus formed by the protomorphic line between
the two, is only, as it wtre, a fold of the deep
surface. If the two layers go on increasing
482
TEGUMENTARY ORGANS.
in this way, however, the ultimate effect will
be that, although growing in reality by its deep
surface as before, the " Lepidine" layer of the
scale will appear to grow by its superficial
surface, and that addition of layers to the
upper surface of the scale observed by Pro-
fessor Williamson, will take place. If the ex-
planation here proposed, however, be correct,
this will form no objection to, but a confirmation
of, Agassiz' views.
It will be well, however, with this clue to
turn from the theory to the facts of scale de-
velopment.
All that I have observed leads me to con-
firm Professor Williamson's conclusion, that
there is no real line of demarcation to be
drawn between placoid, ganoid, ctenoid, and
cycloid scales ; all these forms passing into one
another. Indeed, I conceive that the only
method thoroughly to comprehend the cycloid
and ctenoid scales is to examine, in the first
place, the so-called placoid and ganoid forms.
Hermann Mayer and Leydig have shown
(and the fact is readily verifiable) that the
scales and spines of the Plagiostome fishes are
formed by the gradual deposit of calcareous
matter in processes of the integument, which
are at first coated by the ordinary cellular
ecderon. These diverticula, in fact, originally
resemble other papilla of the skin, and like
them, are bounded by a structureless proto-
morphic layer, marking the boundary between
the cellular ecderon and the enderon.
When the formation of the placoid scale
commences, however, instead of the successive
division and multiplication of the endoplasts
and the cellulation of the periplast of the ec-
deron, which before went on, a deposit of cal-
careous matter takes place at the boundary-
line, and the structureless band remains as
structureless or " basement " membrane, in-
vesting the future spine. The deposit in-
creases until the enderonic pulp occupies but
a very small space, or even completely disap-
pears, and the spine projects as a cylindrical
or conical tubercle. When it has attained
its full length, the deposit does not cease ;
new calcareous matter is continually added to
its inner extremity, but rather in the direction
of breadth than of length, so that, eventually,
an irregular broad plate is formed with the
spine projecting from its outer surface (fig.
308.).
Fig. 308.
once formed, the calcareous additions which
give origin to its base (c) gradually cease to
be in exact apposition with the original
protomorphic zone ; and in proportion as the
base of the spine extends, have we a wider and
wider interval, occupied by the tissue of the
enderon, between its upper surface and the
under surface of the ecderon (/). Examin-
ing it in the perfect state, then, it would appear
that the spine is included in a sac of the en-
deron; and this appearance is very much
strengthened if dilute hydrochloric acid be
added, by which the enamel layer (a) is dis-
solved out, and the structureless membrane
enclosing the spine rendered distinct j while
its continuity with that structureless layer
which bounds the enderon is at once obvious.
From its development, however, it is clear
that this is a simple appearance, and that the
apparent sac results from the projection in-
wards of the extremity of this truly ecderonic
structure. In fact, inasmuch as the base of
the spine grows like its shaft by continual ad-
dition to its inner surface, while its apex is
unquestionably an ecderonic structure, this
base might be considered to be enveloped in
an involution of the protomorphic plane of the
ecderon (fig. 307. c).
Now suppose such plates as these to have
acquired their maximum in width and mini-
mum in height ; furthermore, imagine them to
be so closely set in the skin that the posterior
edge of one over-rides the anterior edge of the
one next behind it, and we have the exact ar-
rangement of the scales in the cycloid and
ctenoid fish (fig. 309.).*
Fig. 309.
It is particularly to be remarked, however,
that the projecting body of the spine being
Scale of the Roach (Leuciscus.)
A, section ; B, surface.
* The flexible cycloid scale of the eel presents an
exact parallel to the tooth-like placoid scale of the
skate, except that it is flat instead of conical, and
that, in the adult state, the scale appears to be com-
pletely included in the enderon, and is wholly
covered by the cellular ecderon. I believe this
appearance of inclusion in a complete sac to pro-
ceed simply from the smallness of the original point
of contact of the scale with the cellular ecderon,
and the rudimentary state in which the whole organ
remains.
TEGUMENTARY ORGANS.
483
A careful study of the scales of that remark-
able animal the Sturgeon, which exhibits in
this, as in so many other characters, its inter-
mediate position between Teleostian and Pla-
giostome fishes, appears to me to throw still
further light upon the difficulties of scale
development.
The scales of the sturgeon are large, slightly
convex, rhomboidal plates, set obliquely in the
skin, so that, while the posterior two-thirds of
their surface are bare and hard, the anterior
third becomes gradually softer from the pro-
longation of the integument over it. The
posterior surface continues hard up to its sharp
edge, but it is supported below by a soft thick
layer of integument, which passes on to the an-
terior soft coat of the scale behind, and thus
masks the real overlapping of this scale by the
posterior edge of that which precedes it
Fig. 310.
Scale of Sturgeon.
A, one of the detached tubercles highly magni-
fied ; B, the entire scale.
The surface of the scale is shining and
glassy. It is marked by a median ridge,
whence it shelves upon each side, and by an
elegant sculpturing produced by raised, hard
ridges of the same nature, which radiate from
the margins centrally, for about a fourth of
the semi-diameter of the scale. In the region
within this zone, the ridges gradually lose
their regularity, the radiating lines anastomo-
sing with one another and forming an elegant
polygonal network. The soft surface of the in-
tegument of the anterior portion of the scale, is
raised into many minute papillae (Jig.3\Q.A,a),
which may be followed for some distance on
to the hard portion. Furthermore, it exhibits
scattered round spots, with projecting centres
of the same appearance as the ridges, and like
them feeling hard to the touch.
If asection of the scale be made (^g.310. B),
its under surface will be found to have a conca-
vity corresponding with the convexity of the up-
per. If the section has passed through one of
the ridges, it is seen that the osseous tissue of
the scale is of two kinds ; a superficial homoge-
neous-looking, dense, comparatively thin layer,
and a deep, thick, laminated portion. If
traced from the centre of the scale to its an-
terior circumference the superficial layer loses
its continuity, breaking up into conical bodies,
which are the sections of the detached calca-
reous spots mentioned above ; the deep layer
thins out, its lamina? gradually becoming
fewer, and leaving a soft membranous space
between their upper surface and the under
surface of these spots. In the centre of the
scale again, a series of rounded apertures are
seen in a tangential section, the sections of
canals which radiate through the scale and
become more numerous and wider towards
its margin. They are connected below with
vertical canals passing through the laminated
layer, and anteriorly they pass into the wide
membranous space above referred to. There
is no histological difference of any importance
in the structure of these two layers ; each is
composed of true bone with radiated cor-
puscles ; the upper being more dense and ho-
mogeneous, the lower less dense and lami-
nated.
If a section be made through several of the
ridges of the upper surface, it will be seen that
they are entirely composed of the hard homo-
geneous osseous tissue. On their sides, how-
ever, and in the valleys between them, more
or less of soft integument remains, whose
pigment masses give the valleys a dotted ap-
pearance. On the other hand, a section of
one of the detached tubercles shows, except
in its consisting of osseous tissue only, that
it is identical with a single spine of the Skate
(j%. 3 1 0. A). It appears to me, therefore, that
there can be no doubt that the ganoid, over-
lapping scale of the sturgeon commences by
an isolated p^coid spine ; that other spines
are developed around this, and their bases
uniting, constitute a placoid scale, between
whose elevations little valleys, bridged over by
the soft integument, remain ; that to the base
of such a plate as this, continual additions of
osseous laminae are made, the radiating Haver-
sian canals being left between the first lamina?
and the superficial plate ; and finally that,
extending in size, the anterior face of this
complex scale becomes over-ridden by the
preceding one. Complicated as it may ap-
pear, it is obvious that all this structure
results from the continued endogenous growth
and union of the primary ecderonic calcareous
deposits, which constitute, as it were, so many
centres of ossification for the large scale. The
final structure, however, is (if we leave out of
consideration its histological character), to all
intents and purposes, that of a cycloid scale ;
and its mode of growth is identical with that
of the large cycloid scale described by Prof.
Williamson.
The increase of the scale is concentric;
addition being made to its posterior, as well
as to its anterior edge and surface ; the only
difference being, that in the latter case the
development of the upper layer is less rapid
than that of the lower, while in the former
i i 2
484
TEGUMENTARY ORGANS.
they are coincident; that soft membranous
separation therefore, which exists between
the two layers anteriorly, is far less developed
posteriorly ; and the soft continuation of the
scale which is flat anteriorly, is inflected pos-
teriorly ; the process of addition being other-
wise the same. Suppose, now, that each
detached calcareous centre of ossification as it
is added to the posterior margin of the scale,
instead of being flattened, were produced into
a spine as in the Rays, then it is perfectly clear
that instead of a cycloid scale, the result
would be a serrated ctenoid scale. And this
appears to be exactly what takes place in
the scales of the perch, according to Prof.
Williamson's description.
From all this, 1 think, we arrive at Prof.
Williamson's conclusion, that fish-scales are
essentially tegumentary teeth ; that like the
latter organs, they result not from the calcifi-
cation of the cellular ecderon covering those
folds of the integument, upon which they are
developed and which correspond with the
dental pulp, but by a calcareous deposit taking
place beneath this, in what represents a deep
layer of the ecderon ; finally that it is, for the
present, an open question whether the deep
layers of all scales are produced by a con-
tinuation of this process, or whether in some
cases a deep truly enderonic structure may be
added to this superficial ecderonic constituent
to constitute the perfect scale. A process of
the latter kind would, at any rate, find its
parallel in the eventual union of the teeth of
many fishes with their jaws, and in that of
the plates of the chelonia with the vertebral
elements.
3. Histology of the tegumentary organs.
Having thus arrived at a general idea of the
mode in which the various forms of integumen-
tary organs are produced from the primary
morphological constituents of every integu-
ment, we have now to consider their minute
histological elements and the mode in which
these proceed from the indifferent tissue of
which all organs are primarily composed.
The tegumentary tissues, like all others, are
produced by the metamorphosis of the pe-
riplast of the protomorphic or indifferent
tissue from which they take their origin, the
endoplasts, to all appearance, taking but little
share in the metamorphic processes. The
chemical metamorphosis of the periplast may
be either into horny, chitinous, calcareous, or
cellulose matter; in form it may become
fibrous, laminated, vacuolated, bony, prisma-
tic, Sic.
As a general rule, the endoplasts tend to
disappear, pari passu, with the metamorphosis
in form and composition of the periplast ; but
the differences presented by different tissues
in this respect have given rise to the esta-
blishment of a distinction between what is
called the process of conversion and that of
excretion. For instance, in the development of
a hair or of a nail, the elements of the pro-
tomorphic layer evidently pass, as such, into
the perfect substance of these organs ; the
periplast simply becoming horny, amfthe endo-
plasts remaining for a long while, or even
always, visible in the cornified tissue. This is
therefore a process of "conversion" of the
protomorphic tissue. On the other hand, the
chitinous coat of the lower Annulosa and the
shells of the lamellibranchiate and gasteropod
Mollusks arise in a totally different manner.
The elements of the protomorphic layer do
not pass into them entire, but they are formed,
like the cuticula of a plant, or like the den-
tine and enamel of the teeth, by the successive
outgrowth of layers of the outer portion of
the periplast. No endoplasts, therefore, are
ever found in them, and there is no conversion
of the protomorphic tissue, but a process of
excretion. *
At first sight this distinction would appear
to be very decided, and likely to afford a good
ground for the formation of definite sub-
divisions of the integumentary organs into
classes. Unfortunately, it is often difficult
in practice to assure oneself in what way a
given tegumentary organ has been formed.
While the presence of endoplasts in a meta-
morphosed tissue is good evidence of its
having been developed by conversion, their
absence is no proof that the tissue has been
developed by excretion ; inasmuch as it may
simply be due to their very early disappear-
ance. In fact, if any one affirm that the shell of
a Unio or of a Crustacean, notwithstanding
the impossibility of detecting endoplasts in its
youngest lamina?, is in reality formed by the
successive apposition of entire layers of the
protomorphic tissue, in which the endoplasts
disappear so early that they cannot be de-
tected, it would be very difficult absolutely to
disprove the assertion, though we might ask
for evidence of its truth. Disbelieving in the
doctrine of the special vital activity of the
endoplasts, I confess the question does not
seem to me to be of much importance, and I
have only enlarged upon the subject because
great weight has by high authorities been laid
upon these distinctions. It appears to me
that the processes of conversion and of excre-
tion grade one into the other, and that no
real subdivisions can be based upon the oc-
currence of either to the exclusion of the
other. I will, however, take care to indicate
what appear to me to be clear instances of
each. I shall now proceed to consider the
histological structure of the integuments of
animals in the following order : 1. Hydroid
and Actinoid Polypes and Beroidae. 2. An-
nulosa, including the Worms and Echinoderms.
3. Mollusca, including the Ascidians and Po-
ly zoa. 4. Vertebrata.
1 . Hydroid and Actinoid polypes. In these
animals the integument consists either of a
simple cellular and vacuolated ecderon, or
the outer layer of this is developed into a
structureless coat, which may become thick-
ened by repeated additions, and thus attain
considerable dimensions. In the common
* Using the word in the sense of " growth out,"
not in the common perverted signification of fluid
transudation and hardening.
TEGUMENTARY ORGANS.
Cainpanularia, for instance, the outer wall
of the bud from which a polype is to arise
consists, at first, of a mass of indifferent tissue.
As development proceeds, the outer portion
of the mass is converted into a structure-
less membrane, which becomes detached from
the body of the polype through its whole
extent, and constitutes the future cell, the
subjacent ecderon taking on the ordinary
cellular structure. On the pedicle the same
process goes on to a less extent, the struc-
tureless layer becoming separated only at
intervals, so that the pedicle acquires a ringed
appearance.
An integument of one or other of these
descriptions is to be met with in all the
Sertularian and Actinoid Polypes, and is
obviously, in these cases, the result of a
process of excretion. In the Medusce and
Beroidce, on the other hand, where the integu-
ment is thick and gelatinous, the ecderonic
tissue is converted, as a whole, into what
closely resembles rudimentary connective
tissue, in which elastic elements and muscular
fibres are developed. The presence of peculiar
organs, called the " Thread or Urticating cells"
constitutes an extremely characteristic feature
in the integument of these creatures. These
JFfe.311.
(,fig. 311.) are composed of a delicate mem-
branous sac (), enclosing a much thicker one
(6), which is open at one extremity, the
aperture being stopped by the end of a more
or less irregular short stiff sheath (c), some-
times giving attachment to several distinct
rays or spines (rf), applied together, which is
fixed to the edges of the aperture, and oc-
cupies the axis of the inner sac. To the ex-
tremity of this sheath a long, frequently
toothed filament is attached (e), and lies
coiled up round the central sheath, and in
close contact with the walls of the sac. The
latter are very elastic, and seem to be tensely
stretched by the contained fluid during life ;
for, on pressure, the sac suddenly bursts, and
its contents are evacuated so rapidlv as hardly
to allow of the process being traced. 1 be-
lieve, however, that the long filament is pushed
out by the side or through the axis of the
central sheath, remaining still firmly attached
to the latter, so that the result is the appear-
ance exhibited in the accompanying figure (c),
where the sac is seen empty, the long serrated
filament being attached to the sheath, which,
everted and with its spines spread out, is itself
fixed to the margins of the aperture. The
violent protrusions of these minute serrated
filaments, aided, perhaps, by some aridity of
the liquid of the sac, is in the larger kinds,
such as those which exist in Physalia, exceed-
ingly irritating to the human skin, and usually
proves fatal to the minute creatures on which
the Hydrozoic and Anthozoic polypes prey.
Integument of the Annulosa. The integu-
ment of the lower Annulose tribes, of young
forms and of the more delicate parts of a
great majority of the higher Annulosa, con-
sists of a thin structureless chitinous mem-
brane developed from the subjacent cellular
ecderon, in a manner essentially similar to
what has been described in the Polypes.
Leydighas particularly described this form
of integument in Entomostracous Crustaceans,
(Branchipus and Argulus) in insect larvae,
(Corethra), and among the Annelids in Pisci-
cola, Nephelis, Haemopis, Sanguisuga, Clep-
sine and Lumbricus, where the integument
consists of two portions a deep cellular layer
and a superficial layer, which is either abso-
lutely structureless, or is fibrillated ; being in
no case formed by the coalescence of the sub-
jacent cells, but by excretion from them.
A similar structureless excreted integument
is found also in Planariae, Nemertidae, in many
Cestoidia, Nematoidea and Trematoda, and,
according to the late researches of Leydig, on
Synapta, in the Echinoderms also. Where
the integument is not very thin, and con-
sists of several layers of chitinous matter, the
added laminae commonly take on a fibrous
structure. The Nematoid worms present par-
ticularly good examples of this complication.
Thus, for instance, the integument of Mermis
albicans, which has lately been examined with
much care by Dr. Meissner, consists of three
layers, the middle of which is double. The
outermost of these layers is either structureless
or presents a distinction into transverse hex-
agonal plates, each of which occupies ^ of the
circumference of the animal. At the head
and tail, small polygonal plate-like markings
replace these, and such small plates could be
detected, making up the large ones. Dr.
Meissner calls them " cells," but expressly
states that he never detected any nucleus in
them, and it seems more probable that they
are produced by modifications of the original
external structureless layer, similar to those
which, as will be seen, occur in the Crustacea
and Mollusca.
The middle substance of this integument is
composed of two layers of fibres one above
the other. The fibres are parallel in the same
laver, but those of the two layers cross one