EXTERNAL CHARACTERS--INTEGUMENT
The form varies enormously, worm-like in some, comparatively short and
heavy, elongate and more or less slender, or extremely gracile and
almost filiform, in others. In this respect our common Grass-snake
occupies a central position, and for this reason is termed a moderately
slender form, anything above or below this standard being described as
comparatively short or elongate. Our shortest and stoutest European
Snakes are the Vipers, especially _Vipera ursinii_; our longest and
slenderest, the _Coluber_ and _Zamenis_, especially _Zamenis dahlii_.
These extremes in both directions are, however, far surpassed by many
exotic snakes, as we find on comparing, for instance, one of the African
Puff-adders (_Bitis_), with certain _Oxybelis_ and _Leptognathus_ from
Tropical America. The body may be somewhat rigid, as in some burrowing
and ground snakes, not unlike in appearance to our Slow-worm and other
limbless Lizards; or extremely flexible, as in many Pythons and Boas and
in the Tree-snakes generally. This flexibility may be accompanied by a
vertical compression of the body in relation with an arboreal existence,
whilst sluggish snakes, such as most of the Viperidæ, may be remarkable
for the flattening of the body, which they may further increase when
basking in the sun or in order to assume a more formidable appearance on
the approach of an enemy. This power of flattening out the whole or the
anterior part of the body is possessed by many snakes, poisonous as well
as harmless, and reaches its highest degree in the Cobras of India and
Africa, the expanded anterior part being known as the "hood," from the
Portuguese name "Cobra di capello."
Thoroughly aquatic snakes are often short and heavy, but some of the
marine forms, or Hydrophids, may be extremely slender, with the
posterior part of the body compressed. In some of these Sea-snakes the
gracility of the anterior part, or "neck," as it has been called,
contrasts very strikingly with the great girth of the body towards the
tail, and suggests a limbless Plesiosaur.
The tail, the part of the body behind the transversely cleft vent, is
most frequently about one-fourth or one-fifth of the total length; but
it may be much shorter, even reduced to a mere stump, as in the
_Typhlops_, or, at the opposite extreme, enter for one half in the
length of the snake, as in the African _Xenurophis_. This organ may
taper gradually to a fine point; or end abruptly, as if mutilated; or
terminate in a horny spine, such as we see in some of the _Typhlops_ or
in the Australian Death-adder, _Acanthophis_, or in a series of horny
segments which are vibrated like a rattle, as in the well-known
_Crotalus_ of America, to which we shall refer again at the end of this
chapter. In some of the burrowing Uropeltidæ, the very short tail is
obliquely truncated, with indurated shields above, and acts as a trowel.
And, finally, the marine snakes of the subfamily Hydrophiinæ are
distinguished by a strongly compressed, oar-shaped tail, with rounded
vertical outline. In a few forms, arboreal or aquatic, the tail is more
or less prehensile.
Males generally have a longer tail than females, and the genital organs,
which are lodged in its base, cause a swelling of that region which
contrasts with the more gradually tapering extremity of the female, thus
affording a means of distinguishing the sexes externally in the majority
of snakes.
The rudimentary hind limbs of Boid snakes, to be mentioned further on in
the description of the skeleton, terminate in a claw-like horny spur,
which appears on each side of the vent in the male, and sometimes also,
though less distinctly, in the female. These spurs are probably of use
in facilitating the pairing, an explanation which appears the more
plausible from the fact that the snakes provided with them have the
copulatory intromittent organs destitute of the erectile spines which
are present in most others.
The head varies in shape as much as the body. Although never actually
compressed, except in the rostral region, it may be very narrow and
elongate, whilst in the opposite extreme it may be strongly depressed,
and so broad behind as to be abruptly defined from the anterior part of
the body, or "neck." This feature is very marked in some of the
Viperidæ, and this has given rise to the incorrect generalization that
poisonous snakes are distinguished from the harmless by a broad and flat
head, notwithstanding the fact that some of the most dangerous, such as
the Mambas, Cobras, and Kraits, have a comparatively narrow or small
head, not or but slightly defined behind, whilst, on the other hand, the
very opposite condition obtains in not a few of the harmless Colubrids.
Leaving the Typhlopidæ and Glauconiidæ aside for the present, snakes
have a wide gape, cleft far beyond the vertical of the eyes, with, when
closed, one or two notches in front for the passage of the protrusible,
bifid tongue. In most snakes this chink is in the lower border of the
rostral shield, capping the tip of the snout, and allows free passage to
the whole tongue; in the Hydrophids, or Sea-snakes, there are two
notches in the lower border of the rostral shield, through which only
the bifid end of the tongue can be protruded. The eyes, varying from
minute to enormous, are usually free from the surrounding shields, and
may move under a transparent cap like a watch-glass, which appears to
represent the lower eyelid of Lizards. The view as to this homology is
derived from our knowledge of various conditions in certain series of
Lizards of the families Lacertidæ and Scincidæ, where we find a
transparent disc appearing like a small window in the movable lower
eyelid, gradually increasing in size so as to occupy the whole of the
lower eyelid, which finally becomes fused with the rudimentary upper lid
and loses its mobility. In _Ilysia_ and in most of the Uropeltidæ, the
transparent disc over the eye is confluent with a thick horny shield of
which it occupies the middle.
The pupil is usually circular or vertical, rarely horizontal. In some
forms it is difficult to decide whether it is round or vertically
elliptic; in others, like the Boas and Vipers, for instance, it is
decidedly vertical, and contracts to the same extent as a cat's. In some
Water-snakes, and in Sea-snakes generally, the round pupil may contract
to a mere dot. The contraction of the pupil is independent on the two
sides.
The snout, or the part of the head anterior to the eyes, may be short or
long, rounded or pointed, depressed or compressed, sometimes projecting
strongly beyond the mouth, turned up at the end, or terminating in one
(_Langaha_) or two (_Herpeton_) long scaly dermal appendages. In some
burrowing forms it is provided with a more or less trenchant horizontal
or vertical edge. When the sides of the snout (loreal region) form an
angle with the upper surface, the angle is termed the "canthus
rostralis," which may be intensified by the loreal region being concave.
The deep pits which are sometimes present on the lips or between the
nostril and the eye (loreal pit) will be alluded to further on under
Sensory Organs.
The nostrils are either lateral, or, in the aquatic forms, directed
upwards, sometimes entirely on the upper surface of the snout.
Most snakes have a longitudinal groove on the chin (mental groove) to
allow for the distension caused by the lateral movements of the rami of
the lower jaw.
In the Typhlopidæ, the head passes gradually into the vermiform body,
and the small mouth is situated on the under surface of the projecting
snout; the head so resembles the extremely short tail, and the mouth is
so similar in shape and position to the vent, which is close to the
posterior extremity of the snake, that such creatures are often believed
by non-critical observers to have a head at each end. The eyes are very
small, and covered over by the semi-transparent head-shields, or they
may be completely concealed. There is no mental groove. It is much the
same with the Glauconiidæ, which have, however, a somewhat less
abbreviated tail. In both, the nostrils often open on the lower side of
the snout, which may be excavated so as to appear hooked in profile, or
may be provided with a sharp cutting horizontal edge.
Snakes are covered with epidermal folds in the form of scales and
shields, the shape and arrangement of which affords important characters
for their classification. Dermal ossifications are absent.
The scales on the body are usually elliptic or lanceolate and imbricate,
forming straight longitudinal and oblique transverse series, and they
are replaced on the belly and under the tail by transverse shields
mostly corresponding in number with the series of scales, and also with
the vertebræ. The body of the Typhlopidæ and Glauconiidæ is uniformly
covered with polished, closely adherent, rounded, overlapping, sub-equal
scales, without even an indication of ventral shields. In some of the
Acrochordinæ, aberrant aquatic Colubrids, the scaling consists, above
and beneath, of small juxtaposed, sometimes spinose granules, the skin
being suggestive of the shagreen of sharks. In the marine snakes of the
subfamily Hydrophiinæ, the ventral shields are often absent or merely
indicated, and the scales are mostly juxtaposed or feebly imbricate,
sometimes tetragonal or hexagonal, and occasionally studded with spinose
tubercles. In the more typical Ophidia the imbricate scales may be long
and narrow or short and broad, with every intermediate step between the
two extremes; smooth or furnished with a longitudinal ridge or keel, or
even several keels; nearly equal in size or with the median or outer
series more or less enlarged, the longitudinal series in odd, rarely in
even number; instead of running in longitudinal series parallel with the
axis of the body, as is the rule, they are sometimes disposed obliquely,
and among those in which we meet with this peculiarity several genera
are further remarkable in having some of the oblique lateral scales
furnished with a serrated keel, to which we shall again allude in the
chapter on Habits, when dealing with the rustling sounds produced by
certain snakes. The number of longitudinal series of scales on the body
varies from 10 (_Herpetodryas_) to nearly 100 (_Python_, _Boa_); in the
European species from 17 (_Contia modesta_) to 50 (_Eryx jaculus_). The
scales are sometimes furnished near the end with one or two shallow
impressions, termed "apical pits," which afford indications for the
distinction of genera and species; unless of a lighter or darker colour,
as is often the case, these pits are not always easy to see, except in a
strong light and with the aid of a powerful magnifying glass.
The ventral shields, also called "gastrosteges," usually occupy the
whole width of the belly; but they may be much narrower--in _Eryx_, for
instance. They are sometimes bent at an angle on the sides, and this
angle may even form a sharp keel, accompanied by a notch in the
posterior border, corresponding to the keel, as in several of the more
arboreal genera of Colubrids. The shields under the tail, termed
subcaudals or "urosteges," are sometimes similar to the ventrals, but
more often disposed in pairs; in certain species or individuals some of
the subcaudals are single, and the others paired. When the number of
subcaudals is given in the descriptions, each pair is reckoned as one,
and the conical or spine-like shield which caps the end of the tail is
not included. These numbers afford important characters for the
definition of species, and sometimes also for the distinction of sexes.
The subcaudals are nearly always much fewer than the ventrals, but the
difference is often not so great in the males as in the females, the
tail of which is usually shorter in proportion to the body. It is
noteworthy that in many species, if the number of subcaudals (C.) be
added to that of the ventrals (V.), the total is nearly the same in the
male as in the female, however much the respective numbers may differ
when taken separately. The following figures may be given by way of
example, taken from British specimens:
_Coronella austriaca_: [M] _V._ 154; _C._ 58 = 212
_Coronella austriaca_: [F] _V._ 165; _C._ 48 = 213
_Vipera berus_: [M] _V._ 138; _C._ 35 = 173
_Vipera berus_: [F] _V._ 144; _C._ 29 = 173
Although this rule is by no means universal, and does not apply at all
to some species, it will be found to hold good in many cases, and is of
interest in showing that the changes that have taken place in the
vertebral column (the vertebræ corresponding in number to the shields),
according to the sexes, have been by a modification of the character of
the segments about the anal region, a conversion of trunk vertebræ into
caudals, or _vice versa_. In dealing with certain species--of Vipers,
for instance--it is important, for systematic purposes, to keep the
counts of shields distinct for the two sexes.
The shield which covers the vent, the anal shield, is either single or
divided into two.
Some snakes have the head covered with scales or small tubercles similar
to those on the body, but in the great majority the lepidosis is in the
form of large symmetrical juxtaposed shields, the shape, proportions,
and number of which furnish some of the most important characters for
the distinction of genera and species. These head-shields belong to two
primarily different types, from each of which all further modifications
may be regarded as derived by alteration in shape or by disintegration.
The first type is that shown by the Typhlopidæ and Glauconiidæ, which is
explained by the figure on the next page.
The rostral, which is usually the largest of the head-shields, extends
to the upper surface of the head, of which it may occupy the greater
part. In the Glauconiidæ, the ocular usually borders the mouth.
As may be seen by a comparison of the first figure with the second, the
arrangement of the head-shields is essentially different from that which
prevails in the Colubrids and the majority of other snakes.
The second type is exemplified by the head of a member of the genus
_Zamenis_.
In the descriptions, temporals 2 + 3 means two superposed temporals in
the first row, three in the second. The internasals and the temporals,
and the loreal and the preocular, are sometimes absent, and the
prefrontal or the internasal may be single. One or two large shields are
in rare cases present behind the parietals, and are called occipital.
[Illustration: FIG. 1--HEAD OF _Typhlops braminus_. (From "Fauna of
British India")
_f_, Frontal; _ip_, interparietal; _l_, labial; _n_, nasal; _o_,
ocular; _p_, parietal; _po_, preocular; _prf_, prefrontal; _r_,
rostral; _so_, supraocular.]
A breaking up into smaller shields takes place in many snakes. In the
Pythons, for instance, the frontal may be divided into two by a
longitudinal cleft, and separated from the prefrontals by small shields.
In some Vipers, such as _V. berus_ and _V. ursinii_, in which the
frontal and parietals, though reduced in size, usually preserve their
primitive condition, the former is normally separated from the
supraocular by a series of small shields, and the internasals and
prefrontals are broken up; in these snakes the small shield or shields
behind the rostral are termed "apical," and those on the upper edge of
the snout are termed "canthals." The shield which, in Vipers, separates
the rostral from the nasal is called "naso-rostral." Allusion has been
made above to the scaly dermal appendages which terminate the snout in
certain genera. Some Viperidæ are furnished with horn-like erect spines
above the eyes or at the end of the snout, which add greatly to their
sinistral appearance.
[Illustration: FIG. 2--HEAD OF _Zamenis ventrimaculatus_. (From "Fauna
of British India")
_cs_, Chin-shields (anterior); _cs´_, chin-shields (posterior); _f_,
frontal; _in_, internasal; _l_, loreal; _la_, labial (upper); _la´_,
labial (lower); _m_, mental; _n_, nasal; _p_, parietal; _pf_,
prefrontal; _pro_, preocular; _pto_, postocular; _r_, rostral;
_sbo_, subocular; _so_, supraocular; _t_, temporals (first row);
_t´_, temporals (second row); _v_, first ventral.]
The periodical shedding of the outer layer of the epidermis in a single
piece, including even the covering of the eye, is one of the most
striking peculiarities of snakes, although paralleled in the Lizards of
the family Anguidæ, to which our British Slow-worm belongs. The skin
becomes detached at the lips, and is turned inside out from head to
tail, without any sort of laceration when the snake is in good health.
These exuviæ are transparent, but often carry a certain amount of
pigment, especially those of the Vipers, in which the characteristic
dark markings are perfectly visible; they usually exceed the length of
the reptile, owing to stretching. In Sea-snakes the epidermis is cast
piecemeal, and sloughing is a longer operation than in ordinary snakes.
In Rattlesnakes each piece of the rattle, or "crotalon," in which the
tail terminates, represents a retained portion of the sloughed
epidermis. This remarkable appendage looks like a number of horny rings,
but it consists in reality of hollow, bell-like pieces, similar to the
terminal one, or "button," each with a circular constriction, in which
the incurved free edge of the following piece fits, thus keeping the
pieces together without impairing the mobility necessary to produce the
rattling sound for which the apparatus is intended. At each exuviation
one bell-shaped horny piece is added. The number of segments in the
rattle is, therefore, not an index to age, as formerly believed; nor is
it to the number of exuviations, for whilst segments are being added at
the base of the apparatus the terminal ones break off and are lost. A
_Crotalus_ sixteen months old may have six pieces to the rattle if there
have been six exuviations and no loss. No rattle appears ever to
comprise more than about twenty pieces, even in old specimens. The size
of the terminal button shows whether it was formed at birth or at any
later period, no growth taking place in the horny tissue.
So far as trustworthy records are concerned, the largest snakes known,
the Malay _Python reticulatus_ and the South American Anaconda,
_Eunectes murinus_, reach a length of 25 to 30 feet. Measurements of
skins must be accepted with caution, as a skin may easily be stretched
to once and a half its real length; in estimating the exact length from
such a stretched skin, it is necessary to deduct the interstitial spaces
showing between the scales, and about one-fourth of the scale to allow
for the overlap. The smallest snake known is 4 inches long (_Glauconia
dissimilis_). The largest European snake (_Coluber quatuorlineatus_) is
reported to reach a length of 8 feet; the smallest (_Typhlops
vermicularis_) does not exceed 14 inches.
KEY TO THE IDENTIFICATION OF THE EUROPEAN SNAKES FROM EXTERNAL
CHARACTERS ONLY
I. Eyes minute, under the head-shields; mouth small, inferior; body
vermiform, covered with uniform scales above and beneath; vent close
to the end of the body, the extremely short tail ending in a small
spine
_Typhlops vermicularis._
II. Eyes very small, with vertical pupil; upper surface of head covered
with small scales; ventral shields much narrower than the body; tail
short, ending obtusely; subcaudals single, or mostly single; scales
smooth or feebly keeled, in 40 to 50 rows
_Eryx jaculus._
III. Eyes small, moderate, or large; ventral shields at least nearly as
broad as the body; tail tapering to a point; subcaudals paired.
_A._ Pupil round; upper surface of head with nine large shields; no
upper labial in contact with the parietal; anal shield usually
divided.
1. Dorsal scales strongly keeled, with paired apical pits; a single
anterior temporal.
_a._ Nostrils lateral; internasals broadly truncate in front.
Scales in 19 rows; normally 1 pre- and 3 postoculars; usually 7 upper
labials, third and fourth entering the eye; ventrals 157-181;
subcaudals 50-88
_Tropidonotus natrix._
_b._ Nostrils directed upwards; internasals much narrowed in
front.
Scales in 19 rows; normally 2 pre- and 3 or 4 postoculars; suboculars
sometimes present; usually 8 upper labials, fourth or fourth and fifth
entering the eye; ventrals 160-187; subcaudals 48-79
_Tropidonotus tessellatus._
Scales in 21 (rarely 19 or 23) rows; normally 1 or 2 pre- and 2
postoculars; usually 7 upper labials, third and fourth entering the
eye; ventrals 147-164; subcaudals 46-72
_Tropidonotus viperinus._
2. Dorsal scales smooth or feebly keeled; normally a single loreal.
_a._ Two or three superposed anterior temporals (very rarely
one); nostril usually between two nasals.
[alpha]. A subocular below the preocular.
* Scales smooth, in 17 or 19 rows.
Two upper labials entering the eye; preocular not in contact with the
frontal; scales with two apical pits; ventrals more or less distinctly
angulate laterally, 160-230; subcaudals 87-131
_Zamenis gemonensis._
Two upper labials entering the eye; preocular usually in contact with
the frontal; scales with a single apical pit; ventrals very distinctly
angulate laterally, 205-218; subcaudals 98-132
_Zamenis dahlii._
** Scales in 23 to 29 rows (usually 25 or 27), with two
apical pits.
Upper labials usually separated from the eye by a series of suboculars;
preocular in contact with the frontal; scales smooth; ventrals very
distinctly angulate laterally, 222-258; subcaudals 77-107
_Zamenis hippocrepis._
Two upper labials entering the eye; preocular not in contact with the
frontal; scales feebly but distinctly keeled; ventrals not angulate
laterally, 195-234; subcaudals 56-90
_Coluber quatuorlineatus._
Two upper labials entering the eye; preocular not in contact with the
frontal; scales smooth or faintly keeled; ventrals not or but very
obtusely angulate laterally, 172-214; subcaudals 50-80
_Coluber dione._
[beta]. No subocular; scales smooth, or faintly keeled on the
posterior part of the body.
* Ventrals more than 200; scales with two apical pits.
Snout obtuse; rostral broader than deep; scales in 21 or 23 rows;
ventrals distinctly angulate laterally, 212-248; subcaudals 60-91
_Coluber longissimus._
Snout obtuse; rostral broader than deep; scales in 25 or 27 rows;
ventrals not angulate laterally, 222-260; subcaudals 68-90
_Coluber leopardinus._
Snout pointed, strongly projecting; rostral deeper than broad, wedged
in between the internasals; scales in 25 to 29 rows; ventrals not
angulate laterally, 201-220; subcaudals 48-68
_Coluber scalaris._
** Ventrals not more than 200; scales mostly with a single
apical pit.
Rostral at least as deep as broad, often wedged in between the
internasals; usually 7 upper labials, third and fourth entering the
eye; scales in 19 (rarely 21) rows; ventrals 153-199; subcaudals 41-70
_Coronella austriaca._
Rostral broader than deep; usually 8 upper labials, fourth and fifth
entering the eye; scales in 21 (rarely 19 or 23) rows; ventrals
170-200; subcaudals 49-72
_Coronella girondica._
_b._ A single anterior temporal; nostril in a single nasal;
scales smooth, with single apical pits, in 17 rows; ventrals
150-191; subcaudals 53-78
_Contia modesta._
3. Scales longitudinally grooved in the adult, in 17 or 19 rows;
two loreals; canthus rostralis strongly marked; frontal very
narrow, in contact with the preocular; ventrals 160-189;
subcaudals 68-102
_Coelopeltis monspessulana._
_B._ Pupil vertical or vertically subelliptic (sometimes appearing
round in _Macroprotodon_).
1. Scales smooth, mostly with single apical pits; upper surface of
head with nine large shields.
Frontal 1-1/2 to 2 times as long as broad; loreal separated from the
eye by the preocular; one upper labial usually in contact with the
parietal; scales in 19 to 23 (rarely 25) rows; ventrals 153-192; anal
divided; subcaudals 40-54
_Macroprotodon cucullatus._
Frontal 1-1/4 to 1-1/2 times as long as broad, much shorter than the
parietals; loreal entering the eye; scales oblique, in 19 or 21 rows;
ventrals 186-222; anal divided; subcaudals 48-73
_Tarbophis fallax._
Frontal 1-1/4 to 1-1/2 times as long as broad, nearly as long as the
parietals; loreal entering the eye; scales oblique, in 19 or 21 rows;
ventrals 203-235; anal entire; subcaudals 54-70
_Tarbophis iberus._
2. Scales keeled, with two apical pits; anal shield entire.
_a._ No pit between the nostril and the eye; upper head-shields
small, if present; nasal separated from the rostral by a
naso-rostral; eye separated from the upper labials by
suboculars.
[alpha]. Snout not turned up at the end; supraocular usually
extending posteriorly beyond the vertical of the posterior
border of the eye; frontal and parietal shields usually well
developed; usually a single series of scales between the eye
and the upper labials.
Snout obtusely pointed, flat above, or with the canthus slightly
raised; rostral usually in contact with a single apical shield, rarely
with two; 6 to 9 upper labials, usually 7 or 8; scales in 19 rows,
rarely 21; ventrals: [M] 120-135, [F] 125-142
_Vipera ursinii._
Snout pointed, with raised canthus; rostral in contact with a single
apical shield; 8 or 9 upper labials; scales in 21 rows, rarely 19;
ventrals: [M] 130-148, [F] 130-150
_Vipera renardi._
Snout truncate or broadly rounded, flat above or with slightly raised
canthus; rostral in contact with two apical shields, rarely with one;
8 or 9 upper labials; scales in 21 rows, rarely 19 or 23; ventrals:
[M] 132-150, [F] 132-158
_Vipera berus._
[beta]. Snout usually more or less turned up at the end or
produced into a scaly dermal appendage; supraocular not
extending posteriorly beyond the vertical of the posterior
border of the eye; frontal and parietals often absent or very
small; 2 or 3 series of scales between the eye and the upper
labials; 9 to 13 upper labials; scales in 21 or 23 rows,
rarely 19 or 25.
Snout simply turned up, the raised portion bearing 2 or 3 scales;
rostral not more than once and a half as deep as broad; ventrals: [M]
134-158, [F] 141-169
_Vipera aspis._
Snout simply turned up or produced into a small appendage, the raised
portion with 5 or 6 (rarely 3) scales; rostral 1-1/2 to 2 times as
deep as broad; ventrals: [M] 125-146, [F] 135-147
_Vipera latastii._
Snout produced into an appendage covered with 10 to 20 scales; rostral
not reaching the summit of the rostral appendage; ventrals: [M]
133-161, [F] 135-163
_Vipera ammodytes._
[gamma]. Snout not turned up at the end; supraocular narrow or
broken up into several small shields; upper surface of head
with small, usually keeled scales; two or three series of
scales between the eye and the upper labials; scales in 23 to
27 rows, usually 25; ventrals: [M] 151-177, [F] 153-180
_Vipera lebetina._
_b._ A pit between the nostril and the eye; upper surface of head
with 9 large shields; nasal in contact with the rostral; third
upper labial entering the eye; scales in 23 rows; ventrals
149-174; subcaudals 31-44
_Ancistrodon halys._
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