BRANCH CHORDATA (_Continued_). CLASS REPTILIA:
THE SNAKES, LIZARDS, TURTLES,
CROCODILES, ETC.
THE GARTER SNAKE (_Thamnophis_ sp.)
TECHNICAL NOTE.--Garter snakes may be found almost anywhere during
the spring and summer months. If possible each student should have a
specimen, but in case it is difficult to get enough snakes two
students can use a single specimen. If garter snakes are rare, take
any other snake. Snakes will live a long time without feeding and
specimens should be kept alive until ready to use. Kill with
chloroform as directed for the toad (p. 5). After completing the
study of the external characters place each specimen in a
dissecting-pan and with a pair of scissors cut through the scales on
the ventral side, passing backwards from the eighteenth to the
fortieth. Pin back the edges of the cut and thus expose the heart.
Through its lower end, the ventricle, insert a large canula; inject
with a fairly large syringe the glue mass which is described on p.
452. This injection will fill the entire arterial system. To inject
the venous system make another cut through the ventral scales,
cutting forward from the anal scale through about forty of them.
Note the injected mass in some of the vessels already filled. Take
one of the large vessels still containing blood and pass two
ligatures beneath it. Get ready a small canula and cut a slit in the
vessel, elevating the head so that the blood will run out as much as
possible. Now wash the blood off, insert the canula in the slit and
tie one ligature about the vessel containing the canula; have the
other ready to tie after the vein has been injected. Use a new color
for the venous system. Leave specimen in cold water for a time until
the injection is hard. Then continue the cut from the anal plate
forward to the lower jaw and pin out the edges of the cut on both
sides in the dissecting-pan.
=Structure= (fig. 122).--Note that the snake is covered with horny
_scales_ somewhat as the fish is. How do these scales differ from those
of the fish? In snakes the scales are not bony, but are true skin
structures. Note the modification of the scales on the head, back, and
ventral surface. Those on the dorsal surface often have minute ridges,
the _keels_. How do the ventral scales differ from the dorsal ones and
others? By a system of muscles these ventral scales are rhythmically
moved and as their posterior edges are pushed back against some
resisting object the body glides forward. On the head note the pair of
_eyes_. Are there eyelids? In front of each eye note an opening. What
are these openings? Thrust a bristle into the opening and see where it
enters the mouth-cavity through the _internal nares_. Does the snake
have external ears? Observe the very long _jaws_ and note that they are
loosely hinged. Examine the inside of the _mouth_. Are there _teeth_? If
so where are they situated, and how arranged? Note that all of the teeth
point backwards. Food is not chewed. When some object of prey, a frog,
or mouse, for example, is seized, the teeth hold it fast to the roof of
the mouth and by a backward and forward movement of the lower jaws it is
gradually drawn into the large œsophagus. What is the character and
situation of the _tongue_? Just behind the tongue note the narrow slit,
_glottis_, opening into the _windpipe_, or _trachea_. Back of the
trachea opens the _œsophagus_.
When the snake is laid open the elongate _heart_ will be conspicuous
in the anterior third of the body. Insert a blowpipe or quill into the
glottis just back of the tongue, and inflate the _lung_, which is a
long, thin-walled bag extending from the region of the heart
posteriorly for two-thirds of the length of the body. There is but one
developed lung, the right; note at the anterior end of the lung a
small mass of tissue, the atrophied left lung. Running forward from
the lung is a long tube composed of incomplete cartilaginous rings,
connected by membrane, the _trachea_. Note the long straight
_alimentary canal_. Distinguish the _œsophagus_, _stomach_,
_intestine_, _rectum_ and the _anus_.
In the region of the lung is an elongated dark-red glandular mass, the
_liver_. The secretion from the liver passes down through the long
_hepatic duct_ to the oval-shaped green _gall-bladder_ and into the
intestine.
TECHNICAL NOTE.--The bile-duct may be injected through the
gall-bladder with some colored injecting mass.
Note that the duct running off from the gall-bladder to the intestine
passes through a pink glandular organ, the _pancreas_. At the anterior
end of the pancreas is a dark-red nodular structure, the _spleen_. The
alimentary canal, the liver and the spleen are all suspended from the
dorsal wall of the body-cavity by a delicate sheet of tissue. What is
this? This condition we have also noted in the toad and fish.
Toward the posterior end of the body cavity are two long, dark-red
glands, the _kidneys_, which are the principal excretory organs of the
body. Through a long, slender tube (the _ureter_) each of the kidneys
passes off its wastes. Where do the ureters open?
Anterior to the kidneys are the reproductive organs. The eggs,
produced by the female snake, after being fertilized, pass backward
through the egg-tubes. During the breeding season these tubes are much
distended. This is due to the presence of the developing eggs, for the
young snakes are hatched in the egg-tubes.
A successful injection as directed in the first technical note will
have filled both arterial and venous systems. How does the general
shape of the snake's _heart_ compare with that of the toad? The heart
consists of two _ventricles_, incompletely separated, and _two
auricles_. In the snake the _conus arteriosus_ is very much shortened
and is not visible. Note two large vessels arising from the median
portion of the ventricle. The one on the left side is the _left aortic
artery_ or _left aortic arch_, while the right gives off two branches.
Where does the anterior one of these run? The main branch, or _right
aortic arch_, passes back to meet its fellow, the left aortic artery,
forming with it the _dorsal aorta_, which runs posteriorly to the end
of the tail. Note the various branches given off by the dorsal aorta
and trace some of them. Arising from the ventricles beneath the two
aortic arches is the _pulmonary artery_, which goes to the lung. There
the blood is purified, after which it is taken up by the _pulmonary
vein_ and carried back to the left auricle, whence it passes into the
ventricle to be mixed with the impure blood from the right auricle.
From the arteries the blood flows to all parts of the body through
fine _capillaries_, bathing the tissues, giving off oxygen and taking
up the carbonic acid gas. From these capillaries it passes into veins
and so back to the heart; from the anterior end of the body through
the _jugular veins_ and from the posterior portion of the body through
the _postcaval vein_. Flowing forward from the tail in the _caudal
vein_, the blood enters the capillaries of the kidneys, where the
waste matter is taken from it. This part of the circulatory system is
known as the _renal-portal_ circulation. From the kidneys the blood
flows through the postcaval vein anteriorly to the heart.
The blood which passes out from the dorsal aorta to all parts of the
alimentary canal is again collected into veins which unite to form the
_mesenteric vein_. This vein runs to the liver, where it breaks up
into capillaries. Thence the blood is carried into the postcaval vein,
which leads directly to the heart. This part of the circulatory system
which collects blood from the alimentary canal and carries it to the
liver is called the _hepatic-portal_ system.
Just in front of the heart will be noted a nodular structure, the
_thyroid gland_, while a little in advance of the thyroid may be seen
a long glandular mass, the _thymus gland_. The functions of these
glands are not certainly understood.
Remove the alimentary canal and muscles from a part of the body and
note that the _axial skeleton_, like that of the other vertebrates
studied, consists of a series of _vertebræ_ placed end to end. Are
there _arms_ or _legs_? Are _shoulder_ and _pelvic girdles_ present?
How many of the vertebræ bear _ribs_? The ribs connect at their lower
ends with the ventral scales. Note the great number of the vertebræ
and ribs as compared with those of the toad or fish. What are those
vertebræ called which bear no appendages or ribs? Examine carefully
the elongated _skull_ of the snake, especially the modified jaws. A
detailed study of the skeleton may be made by referring to the account
of the skeleton of the lizard in Parker's "Zootomy," pp. 130 _et seq._
The nervous system may be worked out in a specimen which has been
immersed in 20 per cent nitric acid. The description of the nervous
system of the toad (see pp. 12-13) will suffice for a guide to the
study of the nervous system of the snake. The special sense organs, as
eyes and ears, should be examined and compared with those of the fish
and toad.
=Life-history and habits.=--The garter snakes are more or less aquatic
in habit and are good swimmers. They are often found far from water,
but in greatest abundance where the cat-tails and rushes grow
thickest. They feed on frogs, salamanders, and field-mice, which they
swallow whole. All the garter snakes are ovoviviparous, i.e., hatch
eggs within the body-cavity. The eggs, often as many as eighteen or
twenty, are enclosed within widened portions of the oviducts during
embryonic existence; when the young are born they are able to shift
for themselves. During cold weather the garter snake hibernates,
hiding then in some gopher-hole, or, in the warmer climates, under
some log or stone, there to lie dormant until the warm days of spring
come, when it resumes activity.
The garter snake sheds its skin at least once a year, sometimes
oftener. This process may be observed in snakes kept in confinement.
For some time before molting the animal remains torpid, the eyes
become milky, and the skin loses its lustre. After a few days it
conceals itself, the skin about the lips and snout pulls away and the
animal slips out of its entire skin. The snake not only sheds the skin
of the body but also the covering of the eyes. Snakes have no eyelids,
as we have already noted, that which represents the eyelid being a
transparent membrane which covers the eyeball.
No species of the garter snake group is poisonous. Sometimes a garter
snake may appear to be vicious, but its teeth are very short and at
best it can only make a small scratch scarcely piercing the skin.
OTHER REPTILES.
The class Reptilia includes the lizards, snakes, tortoises, turtles,
crocodiles, and alligators. Although popularly associated in the common
mind with the batrachians, the reptiles are really more nearly related
to the birds than to the salamanders and frogs. In general shape they
more nearly resemble the batrachians, but in the structural condition of
the internal body organs they are more like the birds. They are
cold-blooded, and breathe exclusively by means of lungs, the forms which
live in water coming to the surface to breathe. They are covered with
horny scales or plates, which with the entire absence of gills after
hatching readily distinguish them from all the batrachians. While most
reptiles live on land, some inhabit fresh water and some the ocean. As
the young have the same habitat and general habits as the adult, there
is no such metamorphosis in their life-history as is shown by the
batrachians. The reptiles are widespread geographically, occurring,
however, in greatest abundance in tropical regions, and being wholly
absent from the Arctic zone. They are not capable of such migrations as
are accomplished by the birds and many mammals, but withstand severely
hot or cold seasons by passing into a state of suspended animation or
seasonal sleep or torpor.
[Illustration: FIG. 123.--A lizard in the grass. (Photograph from life
by Cherry Kearton; permission of Cassell & Co.)]
=Body form and organization.=--The chief variations in body form among
the reptiles are manifest when a turtle, lizard, and snake are compared.
In the turtles, the body is short, flattened, and heavy, and provided
always with four limbs, each terminating in a five-toed foot; in the
lizards the body is more elongate and with usually four legs, but
sometimes with two only, or even none at all; while in the snakes the
long, slender, cylindrical body is legless or at most has mere rudiments
of the hinder limbs. With the reptiles locomotion is as often effected
by the bending or serpentine movements of the trunk as by the use of
legs. Among lizards and snakes the body is covered with horny epidermal
scales or plates, while among the turtles and crocodiles there may be,
in addition to the epidermal plates, a real deposit of bone in the skin
whereby the effectiveness of the armor is increased. The epidermal
covering of snakes and lizards is periodically molted, or, as we say,
the skin is shed. The bright colors and patterns of snakes and of many
lizards are due to the presence and arrangement of pigment-cells in the
skin. Among some reptiles, notably the chameleons, the colors and
markings can be quickly and radically changed by an automatic change in
the tension of the skin.
=Structure.=--In reptiles, as in batrachians, the chief variations in
the body skeleton are correlated with differences in external body
form. In the short compact body of the turtles and tortoises the
number of vertebræ is much smaller than in the snakes. Some turtles
have only 34 vertebræ; certain snakes as many as 400. The reptilian
skull, in the number and disposition of its parts and in the manner of
its attachment to the spinal column, resembles that of the birds,
although the cranial bones remain separate, not fusing as in the
birds. In the snake the two halves of the lower jaw are not fused in
front but are united by elastic ligaments, which condition, together
with the extremely mobile articulation of the base of the jaws, allows
the snakes to open their mouths so as to take in bodies of great size.
All of the reptiles, except the turtles, are provided with small teeth
which serve, generally, for seizing or holding prey and not for
mastication. The poisonous snakes have one or more long, sharp, and
grooved or hollow fangs (fig. 131). In the legless reptiles both
shoulder and pelvic girdles may be wholly lacking; in the limbed forms
both girdles are more or less well developed.
[Illustration: FIG. 122.--Dissection of the garter snake, _Thamnophis_
sp.]
The tongue of many reptiles, notably the snakes, is bifid or forked,
and is an extremely mobile and sensitive organ. The œsophagus is long
and in the snakes can be stretched very wide so as to permit the
swallowing of large animals whole. Reptiles breathe solely by lungs,
of which there is a pair. They are simple and sac-like, the left lung
being often much smaller than the other. In turtles and crocodiles the
lungs are divided internally by septa into a number of chambers.
Because of the rigidity of the carapace or "box" of turtles the air
cannot be taken in the ordinary way by the use of the ribs and
rib-muscles, but has to be swallowed. The reptilian heart consists of
two distinct auricles and of two ventricles, which in most reptiles
are only incompletely divided, the division into right and left
ventricles being complete only among the crocodiles and alligators,
the most highly organized of living reptiles.
The organs of the nervous system reach a considerable degree of
development in the animals of this class. The brain in size and
complexity is plainly superior to the batrachian brain and resembles
quite closely that of birds. Of the organs of special sense those of
touch are limited to special papillæ in the skin of certain snakes and
many lizards. Taste seems to be little developed, but olfactory organs
of considerable complexity are present in most forms, and consist of a
pair of nostrils with olfactory papillæ on their inner surfaces. The
ears vary much in degree of organization, crocodiles and alligators
being the only reptiles with a well-defined outer ear. This consists
of a dermal flap covering a tympanum. Eyes are always present and are
highly developed. They resemble the eyes of birds in many particulars.
All reptiles, excepting the snakes and a few lizards, have movable
eyelids, including a nictitating membrane like that of the birds. With
the snakes the eye is protected by the outer skin, which remains
intact over it, but is transparent and thickened to form a lens just
over the inner eye. Turtles and lizards have a ring of bony plates
surrounding the eyes similar to that of the birds. In addition to the
usual eyes there is in many lizards a remarkable eye-like organ, the
so-called pineal eye, which is situated in the roof of the cranium,
and is believed to be the vestige of a true third eye, which in
ancient reptiles was probably a well-developed organ.
=Life-history and habits.=--Most reptiles lay eggs from which the young
hatch after a longer or shorter period of incubation. Usually the eggs
are simply dropped on the ground in suitable places (although certain
turtles dig holes in which to deposit them), where they are incubated by
the general warmth of the air and ground. However, some of the giant
snakes, the pythons for instance, hold the eggs in the folds of the
body. In the case of some snakes and lizards the eggs are retained in
the body of the mother until the young hatch; such reptiles are said to
be ovoviviparous, because the young, although born alive, are in reality
enclosed in an egg until the moment of birth. Among reptiles the newly
hatched young resemble the parents in most respects except in size, yet
striking differences in coloration and pattern are not rare. But there
is in this class no metamorphosis such as characterizes the
post-embryonic development of the batrachians.
The food of reptiles consists almost exclusively of animal substance,
although some species, notably the green turtles and certain
land-tortoises, are vegetable-feeders. The animal-feeders are mostly
predaceous, the smaller species catching worms and insects, while the
larger forms capture fishes, frogs, birds, and their eggs, small
mammals, and other reptiles.
=Classification.=--The living Reptilia are divided into four orders, of
which one includes only a single genus, _Hatteria_, a peculiar lizard
found in New Zealand. The other three are the Squamata, which includes
the lizards and snakes,[17] distinguished by the scaly covering of the
body, the Chelonia, which includes the tortoises and turtles,
distinguished by the shell of bony plates which encloses the body, and
the Crocodilia, which includes the crocodiles and alligators, whose
bodies are covered with rows of sculptured bony scutes.
=Tortoises and turtles (Chelonia).=--TECHNICAL NOTE.--Obtain
specimens of some pond- or land-turtle common in the vicinity of
the school. The red-bellied and yellow-bellied terrapins
(_Pseudemys_) or the painted or mud-turtles (_Chrysemys_) are
common over most of the United States. (_Pseudemys_ is found south
of the Ohio River and _Chrysemys_ north of it.) They may be raked
up from creek-bottoms or fished for with strong hook and line,
using meat as bait. They will live through the winter if kept in a
cool place, without food or special care of any kind. Observe
their swimming and diving, the retraction of head and limbs into
the shell, the use of the third eyelid (nictitating membrane), and
the swallowing of air.
Examine the external structure of a dead specimen (kill by
thrusting a bit of cotton soaked with chloroform or ether into
the windpipe; see opening just at base of tongue). Note shell
consisting of a dorsal plate, the carapace, and ventral plate,
the plastron, and the lateral uniting parts, the bridge. Note
legs, and head with horny beak but no teeth. Compare with snake.
The examination of the internal structure of the turtle can be
readily made by sawing through the bridge on either side and
removing the plastron. Note the ligaments which attach the
plastron to the shoulder and pelvic girdles. Note muscles
covering these bones. Note just behind the shoulder girdle the
heart (perhaps still pulsating) and the dark liver on each side
of it. Work out the alimentary canal, the trachea and lungs, and
other principal organs, comparing them with those of the snake.
The skeleton can be studied by dissecting and boiling and
brushing away the flesh which still adheres to the bones. The
comparison of the skeleton of the turtle with that of the snake
is very instructive; marked differences in the skeletons of the
two kinds of reptiles are obviously correlated with the
differences in habits and shape of body. Note in the skeleton of
the turtle especially the shoulder and pelvic girdles and limbs
(absent in the snake) and small number of vertebræ and ribs.
Among the common turtles and tortoises of the United States are
several species of soft-shelled turtles (Trionychidæ) with carapace
not completely ossified and both carapace and plastron covered by a
thick leathery skin which is flexible at the margins; the
snapping-turtle (_Chelydra serpentina_), common in streams and ponds,
with shell high in front and low behind and head and tail long and not
capable of being withdrawn into the shell; the red-bellied and
yellow-bellied terrapins (_Pseudemys_), red and yellow, with
greenish-brown and black markings, common on the ground in woods and
among rocks and also near water and sometimes in it; the pond- or
mud-turtle (_Chrysemys_), also brightly colored and usually confined
to ponds and pond-shores; and the box-tortoise (_Cistudo carolina_),
common in woods and upland pastures and readily recognizable by its
ability to enclose itself completely in its shell by the closing down
of the lids of the plastron. All of these fresh-water and land-turtles
except the soft-shelled turtles belong to one family, the Emydidæ, but
have somewhat diverse habits. Most of them are carnivorous, but few
catch any very active prey. While some are strictly aquatic, others
are as strictly terrestrial, never entering the water. The eggs of all
are oblong and are deposited in hollows, sometimes covered in sand.
The newly hatched young are usually circular in shape, and vary in
color and pattern from the parents.
The "diamond-back terrapin" (_Malaclemmys palustris_), used for food,
is a salt-water form "inhabiting the marshes along the Atlantic coast
from Massachusetts to Texas. About Charleston [and Baltimore] they are
very abundant and are captured in large numbers for market, especially
at the breeding season, when the females are full of eggs. Further
north they are dug from the salt mud early in their hibernation and
are greatly esteemed, being fat and savory."
[Illustration: FIG. 124.--The giant land-tortoise of the Galapagos
Islands, _Testudo_ sp. These tortoises reach a length of four feet.
(Photograph from life by Geo. Coleman.)]
Strongly contrasting with the usually small land- and fresh-water
turtles are the great sea-turtles, such as the leather-back, the
loggerhead and the green turtles. Some of these animals reach a length
of six feet and more and a weight of nine hundred pounds, and have the
feet compressed and fin-shaped for swimming. They live in the open
ocean, coming on land only to lay their eggs, which are buried in the
sand of ocean islands. These egg-laying visits are almost always made at
night, and the turtles are then often caught by "turtlers." The flesh of
most of the sea-turtles is used for food, and from the shell of certain
species, notably the "hawk-bill" (_Eretmochelys imbricata_) the
beautiful "tortoise-shell" used for making combs and other articles is
obtained. The common green turtle (_Chelonia mydas_) of the Atlantic
coast is the species most prized for food. It is a vegetarian, feeding
on the roots of _Zostera_, the plant known in New England as eel-grass,
though farther south it is called turtle-grass. When grazing the
turtles eat only the roots, the tops thus rising to the surface, where
they indicate to the turtler the animal's whereabouts. The turtler,
armed with a strong steel barb attached to a rope and loosely fitted to
the end of a pole, carefully rows up to the unsuspecting animal, and
with a strong thrust plunges the barb through its shell, withdraws the
pole, and, grasping the rope, now firmly attached to the turtle's back,
lifts the animal to the surface. Here, with assistance, he turns it into
the boat, where it is rendered helpless by being thrown on its back and
by having its flippers tied. These turtles are also caught on their
breeding-grounds, being found on the sand at night by the turtler,
turned over on their backs, and left thus securely caught until
assistance comes to help get them into the boats.
=Snakes and lizards (Squamata).=--TECHNICAL NOTE.--A snake has
already been dissected and studied. It will be instructive to
compare the external structures, at least, of a lizard with that of
the snake. Specimens of some species of the common swift
(_Sceloporus_) are obtainable almost anywhere in the United States.
The "pine-lizards" of the east belong to this genus. Lizards may be
sought for in woods, along fences, and especially on warm rocks.
[Illustration: FIG. 125.--The blue-tailed skink, _Eumeces
skeltonianus_. (From living specimen.)]
The group of lizards is a very large one, about 1,500 species being
known, but it is represented in the United States by comparatively few
species. Lizards are especially abundant in the tropics of South
America. The strange and fantastic appearance presented by some of
them has made certain species the object of much interest and often
fear on the part of the natives of tropical lands. In those regions
are current extraordinary stories and beliefs regarding the habits and
attributes of certain lizards like the basilisk and chameleon. Lizards
are all more or less elongate and some are truly snake-like in form.
The legs, though usually present and functional, are in many cases
much reduced, and in some forms, as the glass-snake, either one or
both pairs are so rudimentary as to have no external projection
whatever. Although lizards are often regarded as being poisonous, only
one genus, _Heloderma_, the Gila Monster, is really so. All others are
perfectly harmless as far as poison is concerned, and most of them are
unusually timid. They vary in size from a few inches to six feet in
length. Most of them are terrestrial, some arboreal, and some aquatic.
[Illustration: FIG. 126.--The Gila monster, _Heloderma horridum_, the
only poisonous lizard. (Photograph from life by J. O. Snyder.)]
Among the lizards of this country the swifts and ground-lizards are
familiar everywhere. In certain regions the glass-snake or joint-snake
(_Opheosaurus ventralis_) is common. This animal, popularly considered
to be a snake, has no external limbs, and its tail is so brittle, the
vertebræ composing it being very fragile, that part of it may break off
at the slightest blow. In time a new tail is regenerated. It lives in
the central and northern part of the United States, and burrows in dry
places. In the western part of the country horned toads (_Phrynosoma_)
are common, about ten different species being known. These are lizards
with shortened and depressed body and well-developed legs. The body is
covered with protective spiny protuberances, and in individual color
and pattern resembles closely the soil, rocks, and cactus among which
the particular horned toad lives. All the species of _Phrynosoma_ are
viviparous, seven or eight young being born alive at a time.
In New Mexico, Arizona, and northern Mexico the only existing poisonous
lizards, the Gila Monster (_Heloderma_) (fig. 126) is found. This is a
heavy, deep-black, orange-mottled lizard about sixteen inches long.
There is much variance of belief among people regarding the Gila
Monster, but recent experiments have proved the poisonous nature of the
animal. The poison which is secreted by glands in the lower jaw flows
along the grooved teeth into the wound. A beautiful and interesting
little lizard found in the South is the green chameleon (_Anolis
principalis_). Its body is about three inches long with a slender tail
of five or six inches. The normal color of the chameleon is grass-green,
but it may "assume almost instantly shades varying from a beautiful
emerald to a dark and iridescent bronze color."
In the tropics many of the lizards reach great size and are of strange
shape and patterns. The flying dragons (_Draco_) have a sort of
parachute on each side of the body composed of a fold of skin
supported by five or six false posterior ribs. These lizards live in
the trees of the East Indies and "fly" or sail from tree to tree. They
are very beautifully colored. The iguanas (_Iguana_) of the tropics of
South America are commonly used for food. They live mostly in trees,
and reach a length of five or six feet. The monitor (_Varanus
niloticus_) is a great water-lizard that lives in the Nile, and feeds
on crocodiles' eggs, of which it destroys great numbers. It is the
principal enemy of the crocodile. When full grown it reaches a length
of six feet or even more.
About 1,000 living species of snakes are known. Usually they have the
body regularly cylindrical, and without distinct division into body
regions. Legs are wanting, locomotion being effected by the help of
the scales and the ribs. No snake can move forward on a perfectly
smooth surface and no snake can leap. In some forms, such as the
pythons, external rudiments of the hind limbs are present, but do not
aid in locomotion. The mouth is large and distensible so that prey of
considerably greater size than the normal diameter of the snake's body
is frequently swallowed whole. The sense of taste is very little if at
all developed, as the food is swallowed without mastication. The
tongue, which is protrusible and usually red or blue-black, serves as
a special organ of touch. Hearing is poor, the ears being very little
developed. The sense of sight is also probably not at all keen. Snakes
rely chiefly on the sense of smell for finding their prey and their
mates. The colors of snakes are often brilliant, and in many cases
serve to produce an effective protective resemblance by harmonizing
with the usual surroundings of the animal. The food of snakes consists
almost exclusively of other animals, which are caught alive. Some of
the poisonous snakes kill their prey before swallowing it, as do some
of the constrictors. While most snakes live on the ground, some are
semi-arboreal and others spend part or all of their time in water.
Cold-region snakes spend the winter in a state of suspended animation;
in the tropics, on the contrary, the hottest part of the year is spent
by some species in a similar "sleep."
[Illustration: FIG. 127.--A garter snake, _Thamnophis parietalis_.
(Photograph from life by J. O. Snyder.)]
There are so many common snakes in the United States that only a few
of the more familiar forms can be mentioned. The non-poisonous species
of America belong to the family Colubridæ, while all but one of the
poisonous species belong to the family Crotalidæ, characterized by the
presence of a pair of erectile poison-fangs on the upper jaw. Among
the commonest of the Colubridæ are the garter snakes (_Thamnophis_)
(fig. 127), always striped and not more than three feet long. The most
widespread species is _Thamnophis sirtalis_, rather dully colored with
three series of small dark spots along each side. The common
water-snake (_Natrix sipedon_) is brownish with back and sides each
with a series of about 80 large square dark blotches alternating with
each other. It feeds on fishes and frogs, and although "unpleasant and
ill-tempered" is harmless. One of the prettiest and most gentle of
snakes is the familiar little greensnake (_Cyclophis æstivus_), common
in the East and South in moist meadows and in bushes near the water.
It feeds on insects and can be easily kept alive in confinement. A
familiar larger snake is the blacksnake or blue racer (_Bascaniom
constrictor_), "lustrous pitch black, general color greenish below and
with white throat." It is "often found in the neighborhood of water,
and is particularly partial to thickets of alders, where it can hunt
for toads, mice, and birds, and being an excellent climber it is often
seen among the branches of small trees and bushes, hunting for young
birds in the nest." The chain-snake (_Lampropeltis getulus_) of the
southeast and the king-snake (also a _Lampropeltis_) (fig. 128) of the
central States are beautiful lustrous black-and-yellow spotted snakes
which feed not only on lizards, salamanders, small birds and mice but
also on other snakes. The king-snake should be protected in regions
infested by "rattlers." The spreading adder or blowing viper
(_Heterodon platirhinos_), a common snake in the eastern States,
brownish or reddish with dark dorsal and lateral blotches, depresses
and expands the head when angry, hissing and threatening. Despite the
popular belief in its poisonous nature this ugly reptile is quite
harmless. It specially infests dry sandy places.
[Illustration: FIG. 128.--A king-snake, _Lampropeltis boylii_.
(Photograph from life by J. O. Snyder.)]
[Illustration: FIG. 129.--The gopher-snake, _Pituophis bellona_.
(Photograph from life by J. O. Snyder.)]
With the exception of the coral or beadsnake (_Elaps fulvius_), a
rather small jet-black snake with seventeen broad yellow-bordered
crimson rings, found in the southern States, the only poisonous snakes
of the United States are the rattlesnakes and their immediate
relatives, the copperhead and water-moccasin. These snakes all have a
large triangular head, and the posterior tip of the body is, in the
rattlesnakes, provided with a "rattle" composed of a series of partly
overlapping thin horny capsules or cones of shape as shown in figure
130. These horny pieces are simply the somewhat modified successively
formed epidermal coverings of the tip of the body, which instead of
being entirely molted as the rest of the skin is, are, because of
their peculiar shape, loosely attached to one another, and by the
basal one to the body of the snake. The number of rattles does not
correspond to the snake's years for several reasons, partly because
more than one rattle can be added to the tail in a year, and
especially because rattles are easily and often broken off. As many as
thirty rattles have been found on one snake. There are two species of
ground-rattlesnakes or massasaugas (_Sistrurus_) in the United States
and ten species of the true rattlesnakes (_Crotalus_). The centre of
distribution of the rattlesnakes is the dry tablelands of the
southwest in New Mexico, Arizona, and Texas. But there are few
localities in the United States outside the high mountains in which
"rattlers" do not occur or did not occur before they were exterminated
by man. The copperhead (_Agkistrodon contortix_) is light chestnut in
color, with inverted Y-shaped darker blotches on the sides, and
seldom exceeds three feet in length. It occurs in the eastern and
middle United States from Pennsylvania and Nebraska southward. It is a
vicious and dangerous snake, striking without warning. The
water-moccasin (_Agkistrodon piscivorous_) is dark chestnut-brown with
darker markings. The head is purplish black above. It is found along
the Atlantic and Gulf coasts from North Carolina to Mexico, extending
also some distance up the Mississippi valley. It is distinctively a
water-snake, being found in damp swampy places or actually in water.
It reaches a length of over four feet and is a very venomous snake,
striking on the slightest provocation. The common harmless water-snake
is often called water-moccasin in the southern States, being popularly
confounded with this most dangerous of our serpents. The poison of all
of these snakes is a rather yellowish, transparent, sticky fluid
secreted by glands in the head, from which it flows through the hollow
maxillary fangs. The character and position of the fangs are shown in
figure 131. Remedial measures for the bite of poisonous snakes are,
first, to stop, if possible, the flow of blood from the wound to the
heart, by compressing the veins between the wound and heart, then to
suck (if the lips are unbroken) the poison from the wound, next to
introduce by hypodermic injection permanganate of potash, bichloride
of mercury or chromic acid into the wound, and finally perhaps to take
some strong stimulant as brandy or whiskey.
[Illustration: FIG. 130.--The rattles of the rattlesnake; the lower
figure shows a longitudinal section of the rattle.]
[Illustration: FIG. 131.--Dissection of head of rattlesnake; _f_,
poison-fangs; _p_, poison-sac.]
Of the kinds of snakes not found in this country perhaps the most
interesting are the gigantic boa constrictors, anacondas, and pythons.
Pythons are found in India, the islands of the Malay archipelago, and
Australia, while the boas and anacondas live in the tropics of America.
The largest pythons reach a length of thirty feet and some of the boas
are nearly as large. These snakes feed on small mammals such as fawns,
kids, water-rats, etc., and birds. The prey is swallowed whole, being
first encircled and crushed to death in folds of the body. After a meal
the python or boa lies in a sort of torpor for some time. A famous snake
is the deadly cobra-da-capello of India. These snakes are so abundant
and the bite is so nearly certainly fatal that thousands of persons are
killed each year in India by it. Other extremely poisonous snakes are
the vipers (_Vipera cerastes_), which live in the hot deserts of
northern Africa. Over each eye there is a scaly spine or horn, from
which the name horned viper is derived. The most poisonous snake of
South Africa is the large and ugly puff-adder, which puffs itself up
when irritated. An interesting group of snakes is that of the Hydrophidæ
or sea-snakes, which swim on the surface of the ocean by means of their
flattened and oar-like tails. These forms live in the tropical portions
of the Indian and Pacific oceans, ranging as far north as the Gulf of
California, and spend their whole life in the water, "out of which they
appear to be blind and soon die." They are extremely venomous, but are
all of small size, rarely two feet long.
=Crocodiles and alligators (Crocodilia).=--The crocodiles and
alligators are reptiles familiar by name and appearance, though seen
in nature only by inhabitants or visitors in tropical and semitropical
lands. In the United States there are two species of these great
reptiles, the American crocodile (_Crocodilus americanus_), living in
the West Indies and South America and occasionally found in Florida,
and the American alligator (_Alligator mississippiensis_), common in
the morasses and stagnant pools of the southern States. The alligator
differs from the crocodiles in having a broader snout. It is rarely
more than twelve feet long. The best-known crocodile is the Nile
crocodile, which is not limited to the Nile, but is found throughout
Africa. In the Ganges of India is found another member of this group
of reptiles called the gavial. It is among the largest of the order,
reaching a length of twenty feet. The crocodiles, alligators, and
gavials comprise not more than a score of species altogether, but
because of their wide distribution, great size, and carnivorous habits
they are among the most conspicuous of the larger living animals. They
live mostly in the water, going on land to sun themselves or to lay
their eggs. They move very quickly and swiftly in water but are
awkward on land. Fish, aquatic mammals and other animals which
occasionally visit the water are their prey. The gavial and Nile
crocodile are both known to attack and devour human beings, and these
species annually cause a considerable loss of life. But few such
fatalities, however, are accredited to the American alligator.
FOOTNOTE:
[17] By many zoologists the lizards and snakes are held to form two
distinct orders, Lacertilia and Ophidia.