Internal and external anatomy
Warning: contains images from a dissection
Snakes are one of the most extreme extant examples of a specialised body plan. It is thought that snakes evolved in the early Cretaceous period over 100 million years ago from lizards (probably a descendant of modern day varanids). Scientists continue to debate whether snakes evolved limblessness due to a fossorial or an aquatic lifestyle. The bornean Earless monitor Lanthanotus borneensis is assumed to be the current closest link between lizards and snakes as it possesses numerous snake-like features (it has a transparent membrane below its eyelid which allows it to see much like a snake’s brille and lacks an external ear), burrows and is semi aquatic. Whatever the origins of snakes, they show several differences to lizards. Snakes lack an external ear (explained below), have no eyelids, most lack a left lung and have a different skeletal structure (obviously are all limbless but some ‘primitive’ snakes e.g boas retain a pelvic girdle and spikes, vestigial rear limbs on either side of the cloaca).
Snake external anatomy
Skin: The skin of a snake consists of extensions of the epidermis called scales. Snakes retain a fixed number of scales throughout their lives, they simply get larger or change shape as the snake matures. Scales can be either keeled granular or smooth and are made of keratin (the same protein that makes hair in mammals). Smooth scales feel exactly that, granular are rough and keeled have ridges on them. All three types are evolved for their own purposes: smooth are often seen on fossorial species or species in areas of high humidity due to their superior water losing capabilities for example, in the Honduran milksnake Lampropeltis triangulum hondurensis. Keeled scales are beneficial in water retention (desert species) and can aid in camouflage (e.g eyelash vipers Atheris ceratophora). Granular scales are seen less often and the best example of them is the rough skin of the marine sea snakes Hydrophiinae which is used to grip fish. Many snakes have evolved specialised often microscopic structures on their scales or the scales have been evolved with mutations visible to the naked eye. Examples of these specialised scales are a rattlesnake’s tail or the upturned scale on the tip of a hognose Heterodonsnake’s snout. An example seen on every snake is the transparent brille or eyecap which protects the eyes of snakes while still allowing them to see and is formed during the development of the snake embryo where its eyelids fuse. As it grows a snake undergoes a complete moult/shed of its skin known as ecdysis during which the eyes appear a characteristic blue colour- derived from a lubricating substance between the old and new skin that aids in shedding.
Black racer showing characteristing blue eyes, image by muscogeegirl
Senses: Unlike mammals most snakes don’t rely heavily on their vision. The majority of snakes primarily use smell and touch. Also unlike mammals snakes do not rely on their nostrils for smelling; scent particles are picked up by the extended, forked tongue (top picture). The tongue then returns into the mouth and touches the Jacobson’s organ in the roof of the mouth (bottom picture). Depending which fork picked up scent particles the snake can analyse the chemicals and can tell which direction the stimulus came from.
Vision in snakes is generally of moderate effectiveness, with some fossorial species e.g thread snakes Leptotyphlopidae, having poor vision and thought to only be able to tell light from dark while some arboreal species, like the neotropical vine snakes Oxybelis, rely heavily on vision. The majority of snakes respond accurately to movement.
Pythons and pit vipers (subfamily Crotalinae) possess an extra sense: they can “see” heat. Pits located on or between the labial scales are made up of an anterior and posterior air chamber separated by the pit membrane which is connected to the trigeminal nerve. This membrane is heavily vasculated covered in heat receptors that compare the temperatures of the anterior and posterior air chambers. With this these, snakes can detect radiant heat- used for thermoregulation, predator detection as well as prey detection and location. The pit structure of Crotalinae is comparatively more advanced than that of the pythons, which possess smaller, more numerous pits than the Crotalinae.
Labial pits on a Ball python, image by Daniel Maney
You may be interested in reading our slightly more in depth review of infrared vision in boas and pythons by clicking here: infrared vision in snakes.
Snake internal anatomy
We will now cover the internal anatomy of snakes. Please note from this point onwards the article will contain some graphic images of snake postmortems (on a Gopher snake) kindly provided by Trinity exotic pet veterinary practise - see the end of this article for a banner and link to their website.
Those with a weak stomach may wish to look away now.
Snake Respiratory system
Snakes lack a diaphragm so air is drawn in by expansion of the body using its own muscles or by expanding the ribs. Air flows into the glottis (b) which connects to the trachea which then passes air into the right lung. The left lung is completely vestigial in most snakes i.e. it serves no function. In Boas and pythons it is a third of the size of the right lung while in sea snakes the right lung stretches the entire length of the body to aid in buoyancy. This loss of the left lung is due to a snake’s elongated shape. In the diagram, (c) shows the tongue sheath and (a) refers to the palatine vein. The structure of a snake’s right lung consists of the lung and a posterior air sac with little or no blood supply that functions to regulate the pressure within the snake’s body. The lung’s lining contrasts sharply with the intricate bronchial trees of mammals, in reptiles a simple corrugation of the lung’s surface is all that is necessary as illustrated by L (lung) and A (air sac).
Snake Digestive system
Snakes possess a relatively simple stomach. Prey (all snakes are carnivores) arrives through the oesophagus which, unlike mammals, has little musculature and so food is pushed down by bodily movements. The stomach contains folds internally to increase surface area for absorption and digestion. The small intestine is also similarly weakly folded and passes food into the large intestine where it is transported to the cloaca to be disposed of. The liver (L) produces bile, an important digestive enzyme. The intestines are found in the posterior section of the snake in the coelomic fat bodies (fat stores in the last third of the snake’s body). Also located in this fat store are the gall bladder (G) (unusually, separate to the liver unlike most other animals), the pancreas (P) which produces digestive enzymes and the spleen (S) which is an important part of the immune system of snakes.
Snake Cardiovascular system
Snakes posses a three chambered heart: two atria the (upper chamber) and a separate ventricle (the lower chamber). Due to the lack of a diaphragm a snake’s heart can move, allowing it to consume large prey items.
a= Anterior vena cava which brings blood back from the anterior (front) of the snake to the heart
b= Posterior vena cava, bringing blood back from the rest of the snake’s body to the heart
c= right lung
d=trachea entering the right lung
e=the three chambered heart
f= the aorta- the main artery, carrying blood from the heart
The spleen also filters blood, recycling old red blood cells.
Snake Genitourinary system
A snake’s right kidney is closer to its head than its left. They are elongated and filter waste products out of the blood. These waste products then exit via the ureters and cloaca immediately (snakes have no urinary bladder and so cannot store urine. The snake’s gonads (testes in this picture, (T)) are situated in a similar right in front of left fashion, anteriorly to the kidneys ((I) is the intestines). The male possesses a pair of sex organs called hemipenes for sperm transfer to the female (a pair allows sperm transfer either side of the male). Only one is used at a time. They lie posterior to the cloaca (C) as illustrated. Female snakes are generally oviparous (lay eggs). Rattlesnakes (Crotalus sp.) are ovoviviparous which means the female retains eggs and the young hatch internally, gaining nourishment from the egg yolk only. In fully viviparous species e.g Boa constrictor and Green anaconda Eunectes murinus the young are nourished by a placenta as well as a yolk sac.
I would like to give a huge thank you to Rodney of Trinity Vet Centre for kindly allowing us to use these images, below is a banner and a link to their website :)