Caecilian - external anatomy
Amphisbaena - external anatomy
Amphisbaena - head anatomy
Amphisbaena - cloaca
Amphisbaena skull
When people think of 'worms' they think first of invertebrate worms, such as earthworms
and zoologists also use the term primarily for invertebrates. However, if one looks at the
original English usage of the word it was often applied to legless vertebrates, such as
reptilian serpents and various legless fish-like creatures, indeed anything with a long
worm-like body and this body shape seemed to be the main requirement of a 'worm'. I
prefer not to limit English words to modern taxonomic categories and I personally prefer to
use the term 'worm' to describe any legless animal with a long elongated body.

With this definition in mind, we can speak of vertebrate worms. Here I am going to focus on
amphibians and reptiles with wormlike (vermiform) bodies (though one could also include
various eel-like fish). Indeed, some of these vertebrates are remarkably like invertebrate
earthworms in outward form to be hard to tell apart at first glance! This is a remarkable
example of convergent evolution: organisms of different ancestral lineages and forms
adapting and evolving towards a similar life-strategy.
Caecilians - wormlike amphibians

When most people think of 'amphibians' they tend to think of frogs and toads or newts and
salamanders, but many people are unaware that there is a third major group of
amphibians: the Caecilians. There are about 177 species of known Caecilian found in the
circumtropical zone. They belong to the amphibian order Apoda (also called Caecilia or
Gymnophiona).

Caecilan Sensory Systems

Caecilians have various adaptations to a burrowing lifestyle: they are legless with long
elongated earthworm-like bodies; their eyes are covered by bone and not visible
(protecting them during burrowing and restricting their function to light detection since they
are of little use underground) and their skin forms earthworm-like ridges/rings or annuli
which grip the ground during crawling. Some are also aquatic or semi-aquatic.

Caecilians have a number of unusual anatomical features. Since they no longer require
fully functional eyes, the structures associated with eyes in other vertebrates are free to
adopt different functions in Caecilians. They have a pair of short
retractable tentacles
which act as olfactory organs. These tentacles are retracted by the same muscles which
retract the eyeball and close the nictitating membrane (third eyelid) across the eyeball, to
protect the eye. This pair of muscles is called the
retractor bulbi muscles. (These eye
retractor muscles are absent in humans, with at least one reported exception). The pair of
levator bulbi muscles, which normally raises the eyes to enlarge the buccal cavity in
amphibians, is used in Caecilians to move the tentacle sheaths. Furthermore, the channels
into which the tentacles retract are lubricated by secretions of the
Harderian glands.
These glands normally lubricate the nictitating membrane in those tetrapods that have
one. The tentacles are thought to be chemoreceptive and may pick up odourant molecules
by probing the environment. The tentacular ducts open into the
vomeronasal organ, an
olfactory organ which is additional to the nasal cavity. This tentacle-vomeronasal system
might function in olfaction when the nares are closed during burrowing. The
tentacle
apertures
(TA) are borne on papillae just visible towards the corners of the mouth.

Larvae have
lateral line systems on the head and body, consisting of rows of  
mechanoreceptors (called neuromasts) which detect the movement of water, and
electroreceptors (called ampullae) which can detect distortions in the electric field due to
nearby objects which may otherwise be invisible in murky waters. Terrestrial adults lose
these systems after metamorphosis, but aquatic forms retain them.

Caecilians have a pair of ears, but these lack external openings and eardrums. There is
only one ossicle in the middle ear: the
stapes (columella), which extends from the
quadrate bone (see skull diagram below) to the oval window where vibrations transmitted
by the jaws and skull are transmitted to the inner ear inside the auditory capsule. This is, in
part, an adaptation to burrowing or an aquatic life: vinrations in the ground can be easily
transmitted to the bones of the skull, including the quadrate, and transmitted to the inner
ear. The elaborate system of levers, the maleus, incus and stapes, found in mammals, is
ideal for transmitting sounds from less dense air into the denser fluid of the inner ear. The
Caecilian does not require this. The inner ears of frogs and toads contain two
sound-sensitive structures: the basal papilla, which responds to high frequencies, and the
amphibian papilla which responds to low frequencies. Those Caecilians studied only
have the amphibian papilla and are able to hear low frequency vibrations.
Vertebrate Worms
Serpentes (Snakes)

The group of reptiles known as Ophidians include the snakes and their relatives. The
snakes proper belong to the suborder
Serpentes, the serpents. The serpents belong to
the order of reptiles known as Squamates (Scaly Ones) containing the lizards (including
Amphisbaena) and snakes.

Locomotion

The wormlike form of snakes is obvious. Serpentine locomotion is generally by
sideways undulations, throwing the body into S-shaped curves, gaining traction against
the ground alternately diagonally ahead and left and diagonally ahead and right, resulting
in forwards propulsion (rather as in fish, especially eels, except here the substrate is the
surrounding water).

Some snakes, e.g. boas and vipers, can also move in straight lines by muscular movement
of the ventral scales, which are modified to provide grip (
rectilinear locomotion). There
is generally a single row of
enlarged ventral scales on the undersurface of snakes.
Only the ventral skin moves back and forth (cf. the Amphisbaenia in which the whole skin
slides loosely).

Sidewinding is practiced by Rattlesnakes (Crotalus) and other desert-dwelling or coastal
snakes for locomotion over sand and smooth surfaces. In this locomotive mode, the snake
undergoes one-sided undulations, with only small parts of the body in contact with the
ground to gain purchase as the snake advances sideways.

Concertina locomotion is used in narrow passages, such as rodent burrows. The snake
will throw the posterior part of its body into several folds which press against the sides of
the confinement for anchorage. Then it extends the anterior end of the body and throws
this into folds to secure a forward anchorage point and subsequently brings its the rear
end and repeats the cycle.

Sea Snakes have a tail flattened, from side-to-side, for
swimming. Their eyes and
valve-equipped nostrils are also placed near the top of the head. The large ventral scales
are reduced or absent.

The Evolution of Snakes

The evolution of the worm-like bodies of snakes is controversial, but some things are
clear. Generally snakes lack limbs completely, except for the boas and pythons
(considered more ancient in form) in which vestiges of the hindlimbs and their pelvic
girdles remain. External claws may be present on either side of the cloaca (the common
opening of the gut, urinary and genital tracts) which may have some role in mating.

Snakes have several features that have been attributed to a burrowing ancestry, though a
nocturnal habit or an aquatic ancestry can also explain certain of the features. First of all,
let us consider the eye of serpents.
The eyes of snakes differ from both those of lizards and those of mammals. The first thing
to note is the
spectacle (Sp) which is an additional covering formed by the fusion of the
translucent eyelids. (This covering is shed when snakes shed their skin, making their eyes
temporarily cloudy). This is suggestive of a protective function in burrowing. This accounts
for the unblinking eyes of snakes. This makes the surface of the eye dry, tears are
secreted into the
intraconjuctival space (ICS) between the spectacle and the cornea.
The
cornea acts to focus the light and in humans this does the majority of the focusing,
the thin lens being used for fine focus. In snakes, however, the
spherical lens does more
of the focusing. This is a feature of aquatic animals in which the cornea refracts light
poorly since the cornea has little difference in density from water so the lens has more
work to do.

Changing the focus of the lens operates by an unusual mechanism. In mammals, the thin
lens can be pulled thinner to focus on far objects by the suspensory or ciliary ligaments
around the lens becoming tight when the surrounding ciliary muscle relaxes. Contraction of
the circular ciliary muscle slackens the ciliary ligaments allowing the lens to elastically
expand back to its fatter shape to focus on objects near to the eye. (Loss of elasticity of
the lens with age is probably the main cause of long-sightedness, the loss of ability to
focus on objects close by).

Lizard eyes have a different mechanism. Here muscles directly squeeze the lens, by
pressing on a pad around the equator of the lens, to lengthen it for near-to focusing. In
snakes we have a third mechanism: contraction of the
iris muscle (IM) pushes this ring of
muscle towards the back of the eye (downwards in the diagram below) squeezing the jelly
like
vitreous humour (VH) which pushes the eye forwards. This lengthens the path of
light rays between the lens and retina, allowing more convergence or focusing of the rays
for near-to vision. Thus, the lens of the snake eye moves forwards and backwards. This is
perhaps the easiest mechanism for a lens previously adapted to a spherical shape and
thus not thin enough to easily deform. Therefore, the ciliary body (CB) contains no muscle
in the snake eye (in the mammalian eye this has muscle to operate the ciliary ligaments).
The snake eye also has features suggesting that it evolved for black and white vision,
perhaps nocturnal, and then reacquired detailed diurnal vision (and perhaps colour
vision?). Whether capable of colour vision or not, diurnal animals have at least two main
types of photoreceptor in their retinas: rods for low acuity vision in dim light, and cones for
high acuity vision in bright light. Animals with different types of cones can see colour
(humans generally have three different types of cones, each sensitive to a different range
of wavelengths, they are trichromats; whereas birds have four cones contributing to colour
vision and are tetrachromats and so can probably see more colours than humans).
Snakes lack conventional cones and have rods and a peculiar type of cones which are
thought to have been derived from rods. The conus is thought to supply the eye with
nourishment (and is absent from the human eye).

The cones of lizards have yellow oil droplets which filter off blue light before it reaches the
photosensitive pigments. This is to reduce
chromatic aberration (distortion of the image
due to the greater diffraction of short/blue wavelengths of light). Snakes lack these retinal
droplets and have a yellow-tinted lens instead, to filter out the bluest wavelengths of light
before they enter the eye. Human eyes also have lenses containing yellow chromophores
to absorb harmful UV light. Light damage to the human eye may cause an increased
yellowing with age. This does not occur in non-primate mammals, which lack colour vision,
suggesting that the yellow chromophores may have a role in reducing chromatic
aberration as well as protecting the retina from UV light. In pigeons the lower retina has
yellow droplets in a region called the 'yellow field', whilst the upper retina has redder
droplets in a 'red field'. It is thought that these screening pigments may help to enhance
contrast: the yellow pigment making it easier to see an object against the blue sky, the red
pigment for seeing an image against green terrain
(
http://www.pigeon.psy.tufts.edu/avc/husband/avc4eye.htm).

Other Sense Organs

Snakes have no eardrums and no air-filled middle ear chamber. Instead, the stapes
articulates with the quadrate of the upper jaw. The quadrate articulates with the lower jaw
(see diagram of snake skull below) and when the head rests on the ground vibrations are
transmitted through the jaw to the stapes, the other end of which vibrates against the oval
window of the fluid-filled inner ear. Snakes are most sensitive to low frequency sounds,
with the ear being most responsive in the range 150 to 500 Hz, though vibration sensors in
the skin can extend this range.

Snakes (and lizards) have a specialised olfactory organ (in addition to the usual olfactory
systemm connected with the nasal cavity) called
Jacobson's organ. This is a separate
chamber which opens in the roof of the mouth by a pair of openings. Snakes can flick out
their forked tongues, collecting odourants which are then transfered to Jacobson's organ
when the tongue is withdrawn.

Pit-vipers (Crotalines) have a heat-sensitive pit in front of each eye (between the ear and
nostril) e.g. the Rattlesnake (
Crotalus) found in America. Each of these two cavities
contains a richly innervated membrane, the numerous nerve endings responding to
changes in temperature and infra-red radiation flux. They can detect temperature changes
as small as three thousandths of a degree centigrade allowing the snake to strike
accurately at small warm-blooded rodents 1 m away in darkness.

A Question of Size?

The heaviest snake is the anaconda. Individuals may reach about 28 feet (8.5 m) in length
and 44 inches (1.1 m) in circumference and weigh an estimated 500 lb (about 225 kg) or
more. Anacondas (
Eunectes) are boids, they belong to the boa family. The longest snake,
however, is the Reticulated Python (
Python reticulatus) which can grow to about 33 feet
(10 m) but are much thinner and lighter than the Anaconda.

Both Anacondas and Pythons are ancient lineages. They are non-venemous and retain
vestiges of the pelvis but have mobile jaws that can open wide and enlarged ventral scales
for locomotion. Both were originally classified as boids, though nowadays the Pythons are
usually included in a separate family, the pythonids, on account of being quite distantly
related to boas despite their superficial similarities. Some members of the family are small
forms that burrow in sand or leaf litter, but all generally kill their prey by constriction:
squeezing the prey to suffocate it. Clearly the larger forms can tackle larger prey and reap
the rewards of a more substantial meal, which clearly favoured the evolution of large size
in these forms. Smaller forms feed on smaller animals such as rodents. The largest boa
known to have existed was
Titanoboa. Estimating sizes from incomplete skeletons is tricky,
but current estimates place it at around 40 feet (12 m) in length and about 2500 lb (over
one ton) in mass. The larger pythons and boids tend to live partly in trees and partly in
water. The anaconda is especially aquatic, the water assisting the locomotion of these
giant snakes.

The typhlopids are a family of mostly small snakes with vestigial eyes (generally capable of
detecting light and dark only). The largest members reach about 2 feet (0.6 m) in length.
Typhlops is a small earthworm-like snake. These snakes have a modified rostral scale
which overhangs the mouth to form a shovel-like structure for burrowing. The scales are
small and tightly fitting and the teeth are reduced (often being absent on the lower jaw
altogether). They eat arthropods and earthworms. Vestiges of the pelvic girdle are
present. Several other families of snakes are earthworm-like burrowers: the
leptotyphlopids (Thread Snakes) and the uropeltids.

Fangs and Venom

The skulls of two very different snakes are illustrated below:
Respiration in Caecilians

Caecilians can absorb oxygen across their skin and the lining of their buccal cavity, but
most species also have a pair of lungs. The right lung is often larger than the left one, due
to the narrow cylindrical nature of the body. A small terrestrial form found in Guyana lacks
lungs and nares (Wake & Donnelly, 2010). Its body is small and narrow enough to obtain
sufficient oxygen across the skin and the tongue is well vascularised, perhaps for
absorption of oxygen. A large aquatic form lacking lungs has also been discovered. Gas
exchange in the larvae is explained below.

Reproduction in Caecilians

Unusually for amphibians, fertilisation is internal in Caecilians. The male has a retractable
intromittent organ called a
phallodeum (which can be protruded from the cloaca) which he
inserts into the female vent for sperm transfer. The eggs are gelatinous. In some species,
the females lay the eggs and guard them, coiling around them, until they hatch into either
terrestrial or aquatic larvae. Alternatively, some species give birth to live young (they are
viviparous) with the embryos, developing in the oviduct, developing large filamentous gills
perhaps allowing them to respire in the oviduct. Aquatic forms develop leaf-like or sac-like
gills. Embryos in egg-laying forms also develop gills, for example
Ichthyophis kohtaoensis
embryos develop three pairs of gills in the head region. Later in development, the third
hindmost pair of gills are resorbed, to become internalised in an internal gill chamber, the
other two pairs are partially resorbed and then stripped off after hatching. Dunker
et al.
(2000). In other forms, the gills may persist as a transient stage in aquatic larvae.

In some species development is direct, meaning that the larval stage has been lost (or
retained in the egg) so that the animal hatches in a form resembling a miniature adult. In
some of these forms the juveniles are nourished by skin feeding (
dermatophagy) where
the young feed off the skin of the mother! For example,
Boulengerula taitanus is oviparous
with direct development. The brood size of this species is 2 to 9 and the young feed off the
outer layers of the mother's skin which is specially modified. The outer stratum corneum
(dead, squamous (with flat cells), keratinised skin layers) becomes more voluminous in
brooding females, becoming twice as thick and enriched with lipid vesicles. The skin also
contains poison glands to deter predators.

Typhlonectes is a vivparous Caecilian. A female Typhlonectes 500 mm in length may give
birth to up to 9 babies, each up to 200 mm long. The fetuses develop gills but exhaust their
yolk supplies early on in development (by the time they are 30 mm long). Further
nourishment is then provided by the mother (Wake, 1977). The lining of the oviduct
proliferates, nearlyfilling available space in the oviduct with three rows of epithelial tissue,
two lateral and one ventral, with the fetuses occupying the remaining space. The
epithelium also develops glands that secrete a white colloidal
'uterine milk' containing an
emulsion of lipid droplets. When the fetuses exhaust their yolk supplies, they emerge from
their egg membranes and feed on the uterine milk (they possibly stimulate its secretion by
biting the oviduct wall) and supplement this by scraping off epithelial tissue and muscle
fibres and ingest this, along with some blood. They have specialised fetal teeth which are
probably used in this scraping of the oviduct. The fetuses develop a pair of flattened,
leaf-like gills and usually position one above their head and the other along their bodies
and then lie with their gills pressed close to the highly vascularised wall of the oviduct,
suggesting that gas exchange is also occurring between mother and fetus.

Feeding in Caecilians

Caecilians are 'sit and wait predators', feeding on earthworms and other invertebrates.
Uniquely they have two pairs of jaw closing muscles: the levatores mandibulae (
mandible
levators
), anterior to the jaw joint these paired muscles are usual in vertebrates; and the
accessory interhyoideus posteriors (
IHP), a pair of muscles posterior to the jaw joint. The
upper
skull is kinetic, that is capable of movement: the quadrate is joined with the
squamosal, as the
quadrate-squamosal and these two bones, operating as a unit,
articulate with the neurocranium (bones of the skull enclosing the brain) and can rotate
outwards and upwards slightly. This second lever system is thought to compensate for
lateral movements of the lower jaw during opening and closing and to add to bite force.
Having a second lever mechanism is called streptostyly and the whole jaw articulation can
be described as a
streptostylic joint. This whole elaborate jaw operating mechanism
ensures that bite force is maximised over a range of jaw angles, perhaps permitting the
Caecilian to tackle a wide range of prey.
Caecilian skull
N: nasal aperture
The bones of the Caecilian skull are as follows:
Npm: nasopremaxilla (incorporating the tooth-bearing premaxilla)
Mp: maxillopalatine (incorporating the tooth-bearing maxilla)
F: frontal
Sq: squamosal
P: parietal (lit. 'wall')
O: occipital
Pd: pseudodentary (incorporating the tooth-bearing dentary)
OC: ottic capsule (contains the inner ear)
Q: quadrate (articulates with the pseudoangular (PA) of the lower jaw)
S:stapes (part of the auditory apparatus)
PA: pseudoangular (includes the retroarticular process (Rap))
Rap: retroarticular process (for attachment of the interhyoideus posterior accessory jaw muscles)
Another reason for the involvement of the accessory jaw muscles is due to increased
ossification of the skull. The primary jaw-closing muscles, the levatores mandibulae
(mandible levators) have to pass through a space in the skull, called the
temporal fossa,
to reach the lower jaw. In Caecilians, the skull is compact and solid, which is an adaptation
to burrowing as the head is used to excavate burrows. The development of bone restricts
the space available for the mandible levators, in particular the squamosal covers part of
the side face of the skull. The accessory muscles have thus become more important in
assisting jaw closing.

The skull is further strengthened by bony fusion of some of the skull plates to form
compound plates, such as the pseudodentary, which includes the tooth bearing dentary of
the lower jaw. This reduces the number of sutures within the skull, reducing the risk of skull
plate dislocation. Teeth occur on the (pseudo)dentary, premaxilla-maxillopalatine as well as
the vomeropalatine which forms the roof of the buccal cavity.

Locomotion in Caecilians

Caecilians exhibit three different modes of locomotion. When moving over the surface they
use
sideways undulations with the body pushing against and sliding past static points of
the substrate as waves of undulation pass down the body at the same rate that the animal
moves forwards. When burrowing they use a concertina mechanism. Static points on the
body apply friction against the substrate, to anchor the body, as other parts of the body
are extended away from or pulled towards these static points. If the tunnel is narrow, the
animal will have no room to bend its body and in this case the skin holds against the tunnel
wall as the vertebral column bends in a concertina fashion (
internal concertina). This
requires that the skin can move independently of the vertebral column, like a rod moving in
a loose fitting tube. If the tunnel is wider than the body then the third mode is employed:
the
whole body concertinas, with the skin and vertebrae moving together, bends in the
body applying traction against the tunnel wall. (Herrel and Measey, 2010; Summers and
O'Reilly, 1997). As in earthworms, internal hydrostatic pressure is important for burrowing.
This pressure is applied against a cross-helical array of tendons/connective tissue lining
the body cavity and doubles the forward force caecilians can generate when burrowing.
Muscles within this connective tissue apply pressure to the coelom, pushing the animal
forward for head-first burrowing. The compact skull aids in this process.
Amphisbaenia - Worm Lizards
Amphisbaenians (Worm-Lizards) are reptiles that belong to the squamates ('scaly ones')
along with the snakes and lizards. However, they have some superficial resemblances to
Caecilians due to convergent evolution: both have evolved a wormlike burrowing habit.
Most amphisbaenians have no limbs (one genus has forelimbs) and they excavate tunnels
in damp and compact soils. They vary in length from about 10 cm to 1 m. Superficially they
resemble segmented earthworms, since the skin bears a series of pleated rings. Many lack
skin pigment and the eyes are small and inconspicuous. The tail is rounded like the head
and amphisbaenians can crawl backwards, so at a first glance they appear to have a head
at each end like the mythical Amphisbaena.

Amphisbaenian burrows are branching systems of tunnels which amphisbaenians patrol for
earthworms and arthropods, which they eat. The skin is highly permeable to water and
rather similar to amphibian skin. When burrowing the snout is driven into the soil and then
the head moved about to widen the hole. like Caecilians they have compact skulls to assist
in burrowing and there is also a reduction in the number of bony plates to strengthen the
skull (they lack squamosal, jugal and sometimes the postorbital bones). The frontal and
parietal bones strengthen the brain case and the brain is completely encased in bone.
As an example, we shall consider Amphisbaena alba. This species has 198 to 248 body
rings (annuli) plus 13-21 caudal (tail) annuli. In the middle of the body, each annulus has
65 to 85 scales. The scalation of the head is very variable and often asymmetrical. At the
very tip of the snout is a single rostral scale, followed behind by two nasal scales then by
the pair of large prefrontals. Behind these are the frontal scales. On the lower jaw there is
a T-shaped mental scale on the 'chin' folowed by a slightly larger heart-shaped postmental
and a pair of malars (cheek scales). The snout is blunt and slightly dorsoventrally flattened
and oval in cross-section. The upper jaw projects beyond the lower jaw and inserts
between the four supralabial scales of the upper lip. The sixth annulus constricts and
represents the level of the head joint.
The dorsal scales of the body are much narrower than the squarer ventral scales. The
genital region contains the cloaca behind 4 to 10 cloacal pores. this is followed by the first
postgenital row of scales, usually two in number and then by the second postgenital row of
2 or 3 scales, which may further split to give 5 or more scales. this is followed by a row of
12 to 15 postmalars. The row of 10+ precloacal scales are elongated.
Defensive behaviour in Amphisbaenians

In many amphisbaenians the 4th to 8th segments posterior to the cloaca are usually
constricted (this is not the case in
Amphisbaenia alba) to form an autotomy constriction
which allows the animal to willfully and forcefully detach the end of its tail to create a
defensive decoy to distract a predator or escape if the tail is caught. The fracture plane, as
in lizards, is across a vertebra (rather than between vertebrae) along a predetermined
plane of weakness. The broken end heals with some new bone growth, but the tail tip is not
regenerated.

Amphisbaena alba lacks this feature and will defend itself by lifting the head and tail
vertically and swing them in opposite directions, then the tail remains elevated as a decoy
head whilst the head begins to burrow.

Locomotion and habit in Amphisbaenians

During crawling the skin of the Amphsibaenia slides over the underlying tissues. This skin
has been likened to a sock and these movements appear earthworm-like. They crawl
amongst dead trees and in leaf litter, burrowing up to 2 feet (60 cm) down into more
compacted soil. They eat earthworms, slugs and arthropods.
References

Herrel, A. and G.J. Measey, 2010. The Kinematics of Locomotion in Caecilians: Effects of
Substrate and Body Shape.
J.  Exp.  Zool. 313A: 301–309.

Summers, O.P. and J.C. O'Reilly, 1996. A comparative study of locomotion in the
caecilians
Demophis mexicanus and Typhlonectes natans (Amphibia: Gymnophiona). J. of
the Linnean Society
121: 65-76.

Wuster, W. and R. S. Thorpe, 1992. Dentitional phenomena in cobras revisited: spitting
and fang structure in the Asiatic species of
Naja (Serpentes: Elapidae). Herpetalogica 48:
424-434.
Above top: the skull of a python type of snake. Note the absence of fangs and also that
snakes (like lizards) are
diapsids: they have two pairs of fenestrae, or holes for jaw
muscles, upper and lower.

Above bottom: the skull of the rattlesnake (Crotalus). Note the pair of fangs attached to
the shortened maxilla.


Many snakes lack venomous fangs, such as pythons, though some of these snakes may
have a pair of slightly enlarged posterior teeth. These snakes lacking fangs with venom
canals are called
aglyphous. Some may, however, be mildly venomous.

Evidence obtained from studying developing snake embryos ( ) suggests that fangs
evolved towards the back of the mouth, on the maxilla. In some snakes these fangs remain
posterior, e.g. in grass snakes. These fangs are either solid or else have grooves to
develop venom, from venom glands, into the puncture wounds. These 'rear-fanged'
snakes are said to be
opisthoglyphous.

Vipers, pit vipers, cobras and their allies are front-fanged. In the cobras, the maxilla is
shortened and bears only a few other teeth besides the fangs (the
proteroglyph
condition) whilst in vipers it bears only the fangs (the
solenoglyph condition). This
shortening of the maxilla is due to a lack of sonic hedgehog (SHh) expression in the front
of the developing upper jaw of the embryo snake. This brings the fangs forwards. SHh is a
signalling protein used for short-range communication between cells when coordinating
their activity to develop body segments, tissues, organs and limbs.

Spitting Cobras have the ability to spray venom from their fangs, though it is uncertain
whether they intentionally aim for the eyes or rather the centre of any nearest moving
target. The venom acts as a severe irritant and may cause blindness. Vipers (family
Viperidae) and Cobras (family Elapidae) have closed venom canals, but spitting cobras
have smaller openings at the front of the fang through which the venom is sprayed
(Wuster and Thorpe, 1992). Cobras have quite short fangs and close their jaws with the
fangs erect.

The family Viperidae includes the vipers of the Old World and the rattlesnakes and pit
vipers of the New world. They are the most specialised of poisonous snakes. Their fangs
act as hypodermic needles, the venom grooves have deepened and closed over to form
closed canals. These fangs can be long, since they are folded back into a groove along
the upper jaw when not in use. A pair of
pterygoid protractor muscles pull the pterygoid
bones of the skull forwards; the pterygoids then push against the ectopterygoid bones
which rotates the maxillae, erecting the fangs. The jaws can open by almost 180 degrees.
Snake venom contains protein toxins and enzymes which may be neurotoxic or cardiotoxic.

Snakes can swallow large prey whole. A large python can swallow a leopard whole.
Pythons will coil the anterior of their bodies into an S-shape and then strike at prey within
range, gaining a firm grip with their teeth and then coil around the prey and suffocate it by
squeezing. The jaws move alternately in steps as they move along the prey (this involves
maxillary protractions) with left and right sides moving alternately (like a pair of hands
pulling in a rope) with the teeth anchored on one side as the other side advances. Snake
skulls have a number of joints (8 pairs) to permit flexibility. In particular the mandibles are
only connected by muscle and skin, enabling them to separate.

The glottis can be protruded to keep the airway clear whilst prey is being swallowed.
Muscles in the oesophagus squeeze the food along by peristalsis, as in other animals, but
this is often not enough. Side-to-side bending movements of the spine and body help push
the food along by applying pressure with the ribs. The snake may also crawl forwards to
some extent over the food. Neck muscles also assist with passing the prey along the
oesophagus. The fangs are folded back in this process, the other teeth providing grip,
including additional teeth on the roof of the mouth (palatal teeth).

Most snakes swallow their prey alive. The frontal and parietal bones of the skull protect the
brain beneath from the struggling prey. Venomous snakes have a potentially safer
strategy: they strike to inject their venom and then withdraw and track the dying animal's
movements.

Internal Organs

The left lung is usually reduced, often greatly so and may be completely absent. The
trachea has a major role in gas exchange in some species (the tracheal lung).  Sea
snakes can absorb some oxygen from their skin when diving. There is no diaphragm to
assist breathing (as in mammals) and intercostal muscles bring about inspiration and relax
during exhalation. The rearmost parts of the lungs act as air sacs rather than for gas
exchange. The positions of paired organs tends to be asymmetric, with one organ more
forward than the other. The heart is about one quarter to one third down the length of the
body.

Reproduction and Growth

At least some snakes make use of pheromones, with males following the trails left by
females, for example (probably using the Jacobson's organ). In garter snakes
(
Thamnophis) 10 to 20, or occasionally about 100, males may be drawn to a single female
and the males form a mating ball, competing to gain access to the female who releases
pheromones from her skin. Some males adopt a
female-mimicing strategy by secreting
female hormones which can confuse other males, which evidently improves the odds of the
female-mimics gaining access to the real female and mating successfully.

Snakes (and lizards) mate by pressing their cloacas together. The intromittent organs are
retractible
paired hemipenes just behind the cloaca. Each is a blind-ended hollow tube
which enlarges as blood fills sinuses in its walls and then muscles evert it, turning it
inside-out. Only the one nearest the female is used.


Many fresh water snakes, Sea Snakes and most Vipers bear live young (they are
viviparous): the eggs are retained in the oviduct and nutrients transferred from the mother
to the developing embryos. This transition from egg-laying to viviparity appears to have
evolved on numerous occasions in snakes. Parthenogenesis (virgin birth) can occur in
some snakes which otherwise reproduce sexually. Some pythons brood their eggs, coiling
about them and warming them by generating heat with muscular contractions.

Defensive Behaviour of Snakes

When threatened, most snakes will retreat if they are able to, otherwise they will assume a
threatening and defensive posture: rearing up, sometimes with mouth agape and
sometimes accompanied by hissing. Pythons may use their teeth when hunting to initially
strike and lock onto prey, before coiling around and asphyxiating their target. They are
nonvenomous but will strike and bite when threatened. Some experimental evidence
suggests that pythons aim for the eyes, whilst other evidence suggests they simply aim at
the centre of the nearest part of an intruder, which is often the face, meaning that they
have a high probability of striking the eyes.

Some snakes will coil into a tight ball, burying the head in the centre for protection. It is
much harder for predators to grapple such a ball than a stretched out snake. Some
snakes will rear their tails in the air, whilst hiding their heads, so that the tail acts as a
decoy head to draw the attack of a predator to a less vulnerable body part. Older snakes
may have scars on their tails to testify to such attacks.

The rattle at the tip of the tail of rattlesnakes consists of hollow interlocking and articulated
rings (buttons). An additional ring is added at each molt, but the older more posterior rings
occasionally break off. When the snake vibrates its tail

Some snakes use musking: they squirt or smear foul-smelling material from their cloaca
onto a potential predator. This consists of faeces, urine (uric acid) and musk secreted
from anal glands.

Colouration is an important defence for many snakes. For example,  the Copperhead
(
Agkistrodon) is well camouflaged and blends well among leaf-litter. Camouflage can be
more subtle than matching ambient colours and patterns, stripes will break up the contour
of a snake, making it harder to detect (disruptive camouflage). Other snakes want to be
seen: they have warning colouration (aposematic colouring) in the form of bold stripes of
contrasting colours, such as red/white or black/yellow, as in the venomous Coral Snakes
(
Calliophis). Nonvenemous Kingsnakes (Lampropeltis) mimic the warning colours of Coral
Snakes, a trick called Batesian mimicry.

Classification of Snakes

Snakes belong to the order Squamata (Scaly ones) along with lizards, in the class Reptilia.
Snakes belong to the suborder Serpentes which is taxonomically diverse. Some
representative serpent families are:
Snake Skulls
  • Boidae (Boas), e.g. Boa, Titanoboa (extinct);
  • Elapidae (Elapids), e.g. Bungarus (Kraits), Dendrosapis
    (mambas), Hydrophis (Sea Snakes), Naja (Cobras), Ophiphagus
    (King Cobra), Calliophis (Coral Snakes) and Oxyuranus
    (Taipans);
  • Acanthophis (Death Adders);
  • Pythonidae (Pythons), e.g. Python, Morella (Tree Pythons);
  • Viperidae (Vipers and Pit Vipers), e.g. Vipera (e.g. Vipera berus,
    the Adder), Cerastes (Horned vipers), Crotalus (Rattlesnakes);
  • Typhlopidae (Typical Blind Snakes), e.g. Typhlops.
  • Colubridae, the largest snake family to which about half of snake
    species belong, e.g. Coronella austriaca (Smooth Snake), Natrix
    (Grass Snakes) e.g. Natrix helvetica (Barred Grass Snake).