In this article we will discuss about Reptiles:- 1. Origin of Reptiles 2. Ancestry of Reptiles 3. History 4. Evolutionary Gain 5. Adaptive Radiation 6. Habit and Habitat 7. Size 8. Skin 9. Skeleton System 10. Digestive System 11. Respiratory System 12. Circulatory System 13. Nervous System and Sense Organs 14. Excretory System 15. Reproductive System.
Contents:
- Origin of Reptiles
- Ancestry of Reptiles
- History of Reptiles
- Evolutionary Gain of Reptiles
- Adaptive Radiation in Reptiles
- Habit and Habitat of Reptiles
- Size of Reptiles
- Skin of Reptiles
- Skeleton System of Reptiles
- Digestive System of Reptiles
- Respiratory System of Reptiles
- Circulatory System of Reptiles
- Nervous System and Sense Organs in Reptiles
- Excretory System of Reptiles
- Reproductive System of Reptiles
1. Origin of Reptiles:
Reptiles form a heterogeneous group of vertebrates. They are the true land vertebrates that gave up the practice to go back to water to lay eggs. “Things that before swam in the water now went upon the ground” goes the saying. In fact the emergence of reptiles as the true land- dwelling vertebrates offers the greatest dramatic event in the course of organic evolution.
Emerging from the old aquatic environment, the reptiles became successfully adapted to the new regime and as a result evolution within themselves became escalated. The reptiles dominated the globe in their early phase of evolutionary history and are represented today by a few divergent specialised forms (Fig. 8.1).
Reptiles have a dramatic career. They had an obscure beginning in the Palaeozoic, they enjoyed a dominant status in the Mesozoic and then in the Cenozoic they faded out of the drama. It is accepted that the reptiles splitted off during late carboniferous time from Labyrinthodont Amphibia.
The emergence of such reptiles from Labyrinthodont Amphibia was so gradual that, on the basis of skeletal remains alone, it is difficult to ascertain whether some of them are reptiles that have just ceased being reptiles or Amphibia on way to transform to reptiles.
These ancestral reptilian stock during the Permian, underwent an extensive radiation into four or five well- defined groups and in one of these groups lay hidden the progenitor of mammals.
The parameters for distinguishing reptiles from amphibians are the soft parts, and eggs, egg membranes and numerous skeletal features. The information about the eggs or embryos of primitive reptiles are far from complete and it can only be assumed that they were like those of modern reptiles.
The oldest known amniotes egg from lower Permian sediments in North America represents a time long after the establishment of reptiles.
In tracing the evolution of the reptiles from amphibians attention has been restricted to certain skeletal features of reptiles like single occipital condyle, five toes on the front feet, fusion of more than two ribs (one in Amphibia) with the Sacrum.
Some of the primitive reptiles failed to fulfil these features. In fact, these primitive reptiles showed little advancement and were very similar in their skeletal structures to those of Labyrinthodont amphibians. Considering these difficulties in their identifications some authors preferred to call them Amphibious reptiles. But majority of the workers have designated them as stem reptiles.
Stem Reptiles:
The stem reptiles have been grouped together into the order Cotylosauria. They were contemporary of the primitive amphibians of late Pennsylvanian and early Permian times. In the later part of the Permian, however, the amphibians were outnumbered by these reptiles.
2. Ancestry of Reptiles:
Ancestry of Seymouria:
For a long time Seymouria was considered to be a distinguished member of the order Cotylosauria and its structures were evaluated as intermediate between Amphibia and Reptiles.
Adequate fossil remains of Seymouria have been discovered from the upper portion of the lower Permian sediments from Seymour in Texas. Seymouria was a small form about 5.1 cm in length. It had a few reptilian features like Anapsid skull, single occipital condyle, large parietal eye and five digits.
Recently the characters of Seymouria have been re-examined. The joints in the fingers of Seymouria are like Amphibia. It had only one pair of sacral ribs. It had traces of lateral line sense organs on its skull. Moreover, it existed too late in time (Pre-Permian) to become ancestral to reptiles. On the merits of these characters Seymouria is no longer considered as an ideal primitive reptile. It has been sanctioned a lower rank (i.e., with Amphibia).
Ancestry of Limnoscelis:
The work of Romer (1946) suggests that Limnoscelis, a Captorhinomorph Cotylosaurs, is the primitive reptile. The fossil forms of Limnoscelis obtained from the late Carboniferous or early Permian strata of New Mexico illustrates many characters that one might expect to find in a member of the primitive reptilian stalk.
Limnoscelis was about five feet in length and half of this length was donated by the tail. Its skull was of Anapsid type and was compressed from side to side. The otic notch so characteristic of Labyrinthodont had disappeared leaving an indication at the back of the skull of its region of closure.
The pre-maxillary teeth were large and overhung the front teeth in the lower jaw, a specialisation common amongst early reptiles but seldom present in amphibians. Other members of the primitive reptiles are Captorhinus and Hylonomus.
There is every reason to believe that at some early period (Permian) the reptiles split- ted into two divergent evolutionary lines— one of these lines are called Therapsida which led to mammal-like reptiles and mammals, while the other line is called Sauropsida which led to other reptiles and birds.
Recent investigations suggest that both Therapsid and Sauropsid evolved from Captorhinomorph Cotylosaurs. The position of Ichthyosaurs and Plesiosaurs is still problematical. The Chelonia, derived from common Captorhinomorph Cotylosaurs, is not closely related to other modern reptiles.
No authorities are yet unanimous about the precise stage in evolution at which the Therapsida and Sauropsida lines split off from one another. Though the fact remains that such dichotomy is a must. The brain of a mammal is fundamentally different from that of a modern reptile and could have in no way evolved from the modern reptiles.
Similarly the great arteries from the heart of mammals cannot be derived from that of modern reptiles. According to Goodrich the hook- shaped fifth metatarsal among the Sauropsid (absent in Therapsida) is a useful parameter in distinguishing the two evolutionary lines.
There is no doubt that some of the reptiles living today are linked by a beautiful series of gradations to very ancient progenitors—the Crocodilia, Chelonia and Sphenodon lead back as far as Triassic. But there is no reptile that has persisted from age to age as Ceratodus has done amongst the fishes.
Thus from the point of view of evolution, the living genera of reptiles are never intermediate between amphibians and the mammals or amphibians and the birds. To find a common ancestry for a mammal and a lizard one is to retreat back to an ancient reptilian or more possibly to an amphibian stage in evolution.
3. History of Reptiles:
The name reptiles was given with an eye to the snakes which crawl (repere = to crawl). That reptiles are familiar to man is evidenced from the fact that there are frequent mentioning of snakes, turtles or alligators in eastern and western mythologies.
According to Hindu pantheon the supreme God Vishnu had his third incarnation in the form of a turtle. Vashuki, a great snake on which the earth rests according to Hindu mythology, was used to churn the ocean to bring Amrita, a liquor for immortality.
The snakes have been assigned an evil role in Judeo-Christian tradition. Adam was driven out of the Garden of Eden because of the temptation instigated by a serpent. Crocodiles and snakes are worshipped in Egypt. In Mexico a feathered serpent is the image of a fair God. Pliny and Galen in their time prescribed that to cure various ills vipers may be eaten. Viper tooth was much in use in Europe until very recently.
4. Evolutionary Gain of Reptiles:
The evolutionary gain of the reptiles over the amphibians is the attainment of lungs and the development of extra embryonic membranes, called amnion and allantois. The reptiles and their descendants, the mammals and birds do not breathe by gills at any stage of their life but depend solely on lungs which are effective organs to derive oxygen from atmosphere.
Again the reptiles and other conquerors of the land do not cradle their young ones in open water as do the amphibians and fishes. This has necessitated the formation of a protective antenatal robe round the growing embryo.
The protective robe is the amnion, a thin enveloping sac filled with secreted watery fluid in which the embryo floats. The allantois, on the other hand, is an outgrowth from the posterior part of the digestive tract of the growing embryo.
The amnion helps in withstanding exposure to dry air and protects from mechanical shocks. Allantois is a richly vascular emergency organ which helps in respiration and excretion. Because of the presence of amnion the reptiles, birds and mammals are collectively called Amniota while all the lower vertebrates in which no amnion develops are called Anamniota.
Other features that stand out sharply in reptiles are the presence of a thick integument covered with scales and lengthening and strengthening of limbs for locomotion on land. However, it cannot be denied that the name reptile was suggested with an eye to the limbless crawling snakes.
But in reality the class Reptilia includes not only the snakes but also the lizards, turtles, tortoises, crocodiles as well as the living fossil, Sphenodon of New Zealand. The class Reptilia also includes a large number of extinct forms that dominated the earth during Mesozoic era.
5. Adaptive Radiation in Reptiles:
One of the many features that make the study of reptiles interesting is what Osborn has called adaptive radiation. By the term adaptive radiation is meant the differentiation of animal form and construction which, from a common starting point, follows lines of adaptations in diverse directions in response to the different needs demanded by different kinds of environment.
The ancient reptiles gave trials to all sorts of haunts and ways of living. Some of them like Pterosaurs became aerial, the Ichthyosaurs and Plesiosaurs were aquatic. Amphibious were the Sauropod Dinosaurs. Cursorial adaptation was exhibited by many dinosaurs. It is assumed that some of the mexozoic reptiles became adapted to fossorial and arboreal life.
Adaptive radiation is a dynamic aspect of animal existence and as animals branch out into each available environment they assume characteristic adaptations to meet their specific needs—a fact noticeable in the modifications associated principally with locomotion.
The noteworthy feature in the radiation of reptiles is the fact that most of their adventure specially on land was successful as there was no vertebrate competitor.
6. Habit and Habitat of Reptiles:
Tropical and subtropical regions of the globe are the most suitable abode of the reptilian fauna. The reptile population decline markedly towards the poles and high altitudes.
Reptiles show adaptation to a variety of habitats. Turtles and snakes prefer to live in humid regions. Crocodiles live in swamps or rivers or along the sea coasts while large turtles live in the ocean. Land tortoises live on arid oceanic islands but box turtles are found in forests.
Most of the lizards and snakes are terrestrial but some of them climb rocks and trees. Many lizards, like Geckos take shelter in crevices of rocks, trees and buildings. Some tropical reptiles, like Draco, are arboreal and violent forms. Black snakes often ascend trees in search of food. Snakes live in burrows dug by rodents but many lizards and snakes can dig burrows in sand with the rostral plate situated on the snout.
Since reptiles are cold blooded animals, fluctuation of environmental temperature influences them to a good extent. In tropical countries reptiles remain active throughout the year. In tropical countries reptiles remain active during day time. Reptiles living in the deserts or semi-desert regions show activity in the morning and late afternoon and always avoid the hot mid-day sun.
Many desert snakes are nocturnal in habit. In cold seasons most reptiles undergo a period of dormancy. Lizards and snakes assemble in number in caves or burrows to hibernate. Freshwater turtles go to the bottom of ponds.
7. Size of Reptiles:
Some of fossil reptiles were the largest terrestrial animals ever lived. They were more than 37 metres in length. Largest among the present-day living lizards is the Komado dragon (Varanus komodensis) which is about 3 metres in length.
Among the snakes Anaconda of South America attains a length of 10 metres. In contrast to that the size of the snake, Leptotyphlops of Syria is like that of a sewing needle. Some leather back turtles are 2.1 metres in length.
The snakes vary greatly in length. The largest living snake is probably the reticulated python, Python reticulatus of India and Malaya. It may attain a length of 10 metres.
The South American boa, Eunectes murinus has been recorded to attain a length of 8.5 metres and weighing more than 130 kg. The biggest poisonous snake is the Indo-Malayan King Cobra, Ophiophagus hannah which becomes 5.5 metres long. The longest sea- snakes may grow to a length of 2.5 metres.
8. Skin of Reptiles:
Reptilian skin is dry and is covered by scales derived from the Malpighian layer of epidermis. These scales are horny epidermal structures and are different in origin and structure from the bony dermal scales of fish. The scales vary greatly in form and size. These are small papilla like in Geckos and Chameleons.
In lizards and snakes these are arranged in an imbricate fashion. Scales are large, horny and plate-like in tortoise. The scales often become modified in the form of crests and spines. In most reptiles the scales and the outer layer of the skin is shed periodically. The skin is shed as a whole by snakes, but in others, it is flaked off the body in small pieces.
The rattle snakes are characteristic in having a rattle in the tail. A rattle (Fig. 8.67) is a ring like horny structure of stratum corneum that lies at the end of the tail, formed by a series of loosely connected horny segments.
When the animals, specially the large mammals move towards them these segments make a sound like a hand rattle. A rattle snake sheds its epidermis periodically except at the tip of the tail where the horny segments add a “bottom” each year.
Mucous glands which were so abundant in amphibians, are absent in reptiles. Glands derived from the mucous glands are restricted to some special parts of the body. The femoral glands of lizards and the glands on the throat and jaws of crocodiles are believed to be derived from the mucous glands. But the role of the glands has been changed. They are no longer used in keeping the body moist but are used as scent glands.
Many reptiles can change their colour due to threat, courtship or for concealment. For this purpose many reptiles contain pigment in the skin. In the dermis the pigments are present in distinct cell known as melanophores but in the epidermis the pigments remain scattered between the cells. The pigment cells function under the control of nervous system and in some forms remains under hormonal control.
9. Skeleton System of Reptiles:
Reptilian skeleton is strongly built with solid vertebral centra. The skull is hinged upon the atlas by a single occipital condyle. The skull bones are firmly ankylosed and are with vacuities. The mandible is made up of several pieces of bones. Table 37—Chordata shows the different bones in head skeleton according to their nature and mode of origin.
The teeth are simple, conical and have acrodont, plurodont and thecodont types of attachment with the jaws. A pineal foramen is present in the roof of the skull. The vertebral column of the reptiles is divisible into five zones.
Sacral vertebrae are usually two in number and caudal vertebrae bear chevron bones. Ribs are single headed and may be present on thoracic and abdominal regions. The sternum bears a minimum of two ribs. Pectoral and pelvic girdles are stout except in snakes and the limb bones are well-developed.
Characteristic Features of Skull.
A reptilian skull is characterised by the following:
(a) Lack of cartilaginous elements.
(b) Presence of a single occipital condyle.
(c) Presence of parietal and coronary sutures.
(d) Presence of a parietal foramen in between the parietal sutures.
(e) Presence of an inter-orbital septum except snakes and limbless lizards.
(f) Presence of a rod-like epipterygoid bone between pterygoid and parietal.
(g) Presence of ectopterygoid or trans-palatine between pterygoid and maxilla.
Besides these features, there are certain extra apertures or fossae in the reptilian skull. The classification of the skull of reptiles is based on the presence of these fossae.
Nomenclatures of Reptilian Skull:
(A) Synapsid (Fig. 8.68A):
Skull with a pair of lateral or inferior fossae and situated beneath the junction of squamosal and post-orbital involving the jugal.
Example:
Therapsida.
(B) Parapsid (Fig. 8.68B):
Skull with a single temporal vacuity and differing from synapsed type by the fact that the postorbital and squamosal meet below the opening.
Example:
Ichthyosauria.
(C) Anapsid (Fig. 8.68C):
Most primitive of the reptilian skull, in which the roof and the sides of the skull are entire, i.e., there is no fossa.
Examples:
Seymouria and Chelonians.
(D) Euryapsid (Fig. 8.68D):
Skull with a single fossa behind the eye and bounded below by postorbital and squamosal.
Example:
Protorosaurs.
(E) Diapsid (Fig. 8.68E):
Skull with both superior and inferior fossae. The fossae are located on the temporal region and postorbital and squamosal meet between them. This type of skull is most specialised and most surviving reptiles possess diapsid type of skull.
Significance of Arcades and Fossae:
a. Significance:
The transition of reptilian skull from primitive anapsid type to present day diapsid type stands in a good way in interpreting the evolution of reptiles.
b. Advantages:
The primary purpose of the skull or brain box is to house and protect the brain. It also affords a platform for the attachment of muscles. In reptiles, the skull is not large as the brain itself is relatively small. The little increase in the size of the skull of reptiles is only for housing the muscles which work the lower jaw.
This little increase has again been compensated by the formation of fossae or in other words reduction of part of skull bones. In reptilian skull, a happy compromise between strength and heightness and at the same time economy of material has occurred. Figs. 8.69 and 8.70 illustrate the skull of two lizards, Uromastix and Varanus.
Table 38 shows the nomenclature with characteristic features of reptilian skulls (after Parker and Haswell, 1964).
The vertebral column is differentiated into different regions. The regions are: cervical, thoracic, lumbar, sacral and caudal. In snakes, the vertebral column is differentiated into precaudal and caudal regions. Fig. 8.71 shows the different vertebrae in a typical lizard, Varanus.
10. Digestive System of Reptiles:
Animals from fish to mammals constitute the food of most reptiles. Tortoise, turtles and some lizards like Chukwala and Iguanos of deserts feed on vegetation. The Iguanos of Galapagos dive deep into the sea to obtain marine algae for their food. The total intake of reptiles is small compared with that of birds and mammals. Food is captured by teeth. Chameleons capture their prey with the help of their elongated tongue.
Feeding Mechanism in Snakes:
Non-poisonous snakes during food-getting coil round the body of the prey until the prey dies from asphyxiation. Poisonous snakes strike with their fangs to kill the prey which dies from asphyxiation. They also strike with their fangs to kill the prey by injecting venom.
The snakes are often capable of swallowing animals much larger than the size of its own body. There are several structural adaptations for this purpose. Swallowing depends on the condition of teeth and co-ordinated system of movements of the two rami of the lower jaw. The process of swallowing is slow and quite laborious.
Digestive canal begins in mouth and ends in anus. The edges of the jaws are provided with teeth. Teeth are present on some of the bones of the palate in some cases. The teeth are conical and recurved backwards. Teeth may be serrated or modified to form crushing pads.
Fangs are modified teeth. Excepting the lizards, succession of teeth is a continuous process in all reptiles. The tongue is hardly movable in crocodiles but long, forked and highly movable in snakes. Salivary glands are present in majority of the reptiles. In some snakes and Heloderma, the upper labial glands become modified into poison glands.
The alimentary canal is typically of vertebrae type. The stomach is tubular, small intestine is rather short. The large intestine is wide and leads to a short caecum. The cloaca is subdivided into three incomplete chambers. The upper chamber or coprodaeum is meant for faeces.
The middle chamber or urodaeum receives the ureters and the gonoducts. The lower chamber is called proctodaeum which terminates in a cloacal sphincter. The division of the cloaca has become a necessity for the retention of water. It is believed that the coprodaeum and urodaeum reabsorb water from both faeces and urine.
11. Respiratory System of Reptiles:
The external nares, internal nares, mouth cavity, glottis and larynx constitute the respiratory passage. The respiratory passage leads to a pair of lungs. The larynx is supported by cricoid and arytenoid cartilages. There occurs considerable variation in the structure of reptilian lungs. In Sphenodon lungs are like sacs, the walls of which bear small pockets lined with respiratory epithelium.
In lizards and chelonians the lungs are provided with deeper pockets forming definite alveoli. In some lizards pockets assume long channels leading from the lung cavity to the wall of the lungs. In reptiles, the wall of the lungs is provided with muscles which contract and expand rhythmically to assist in ventilation.
The hinder part of the lung in many reptiles is smooth. The hinder part is more a reservoir than a respiratory surface. In chameleons, the cavity of the lung is drawn outside the wall of the lungs to form air-sacs like that of birds.
Mechanism of Respiration:
Though there occurs no significant respiratory activity through skin and epithelium of mouth in reptiles, yet it has been shown that in Trionyx, oxygen is taken up by the epithelium of pharynx. The urodaeal part of the cloaca has a paired diverticula in many turtles. These diverticula become filled and emptied rhythmically with water coming through the anus. These are regarded as accessory respiratory structures.
In reptiles, expansion and contraction of the thorax by muscular activity draws air in and drives air out. The cycle of breathing in lizards is known. It consists of three phases. In the first phase there is an expiration. In the second phase there is an inspiration and in the third phase there is expiration.
The glottis is held open during the cycle up to the end of inspiration. The air is driven out of the lung in the first expiration and sucked in during inspiration. The glottis remains closed during second expiration. The cycle of breathing differs in various reptiles.
In chelonia, because of the immovable carapace, expansion of the general wall of the thorax is difficult. But the lungs are expanded by the movement of head, limbs and girdles.
In crocodiles, a septum across the abdominal cavity divides the abdominal cavity into anterior part housing the lungs and a posterior part containing the viscera. The septum is not muscular and probably has no role in the respiratory mechanism and as such it cannot be regarded as homologous with the mammalian diaphragm.
12. Circulatory System of Reptiles:
Heart:
The reptilian heart consists of two distinct auricles and a ventricle which is internally divided by an inter-ventricular septum into left and right portions. In chelonia, snakes and lizards the septum is well-developed but does not close off the left-hand portion of the ventricle from the right-hand portion.
The left-hand portion is larger than the right-hand portion. The left portion is further divided imperfectly into a cavum venosum to the right and a cavum arteriosum to the left by septum. This secondary septum is formed by the fusion of trabeculae.
In crocodiles the cavity of the ventricle is completely divided into left and right portions. The right and left systemic arches cross each other in crocodiles and at the point of contact of the wall of the left and right systemic arches, there is an aperture, called foramen of Panizza. Although this foramen places their cavities in communication, mixing of oxygenated and deoxygenated types of blood does not occur.
The conus arteriosus is absent in reptiles and the arterial arches arise directly from the ventricle. Sinus venosus has shown a tendency to become fused with the right auricle in varying degrees. Excepting in Sphenodon the presence of sinus venosus is not properly distinguishable.
The right auricle receives venous blood from the body and the left auricle receives oxygenated blood coming from lungs. The cavity inside the right ventricle is known as cavum pulmonale. From the cavum pulmonale arises the pulmonary artery and from the cavum venosum the left and right aortic arches originate.
Mechanism of Circulation:
The contraction of the auricles pours venous blood from the right auricle into the cavum venosum and arterial blood from the left auricle into the cavum arteriosum. The cavum pulmonale becomes filled with venous blood and some amount of this venous blood overflows into the cavum arteriosum past the edges of the incomplete inter-ventricular septum.
When the ventricle contracts, the lip of the inter-ventricular septum touch the ventricular wall above and thus the cavum pulmonale is cut off from the rest of the ventricle. With further contraction of the ventricle the venous blood of cavum pulmonale is forced to go to the lungs through the pulmonary artery.
The remaining portion of the blood in ventricle is mixed in nature because it contains both arterial and venous blood. This mixed blood with the contraction of the ventricle passes out through the aorta.
The root of the left and right aortae inside the ventricle is so oriented that the right arch receives mostly the arterial blood and the left arch receives mixed blood. Thus the mechanism of circulation of blood from the heart in reptiles is so designed that even in the absence of anatomical division of the ventricle there has occurred a physiological separation of oxygenated and deoxygenated blood.
Arterial System:
Of the six pairs of embryonic arterial arches, the 3rd, 4th and 6th pairs are retained in reptiles. The 5th arch is present in reduced form in some reptiles. The remnant of the radix of aorta between 3rd and 4th arches is present on each side in some snakes.
The ill-defined conus arteriosus is splitted into three vessels. The fourth arch on the left side arises from the right side of the partially divided ventricle. The fourth arch on the left side becomes the left aortic arch. The fourth arch on the right side arises from the left side of the ventricle.
It establishes a connection with a portion of the radix of aorta of the right side and becomes the right aortic arch. The common carotid arch arises from the right aortic arch and becomes divided into external and internal carotid arteries. The 6th aortic arch loses all its connection with radices of aorta and becomes the pulmonary arteries.
The radix of aorta between carotid and systemic arches is present as ductus caroticus in many lizards. Similarly a part of the radix may remain connected with the 6th aortic arch. This connecting part is known as ductus arteriosus. Ductus caroticus and ductus arteriosus are both present in Spehnodon.
Venous System:
The venous system in reptiles shows little advancement over that of amphibians. Two Percival’s and one postcaval open into the sinus venosus which itself shows a tendency towards fusion with the right part of the auricle. Anterior abdominal vein is prominent in reptiles and more blood passes through it from the posterior part of the body. The renal portal system loses its importance to some extent.
13. Nervous System and Sense Organs in Reptiles:
The central nervous system in reptiles shows a better developed condition than the amphibians. In reptiles the cerebellum and the cerebral hemispheres are more developed than the amphibians. The cerebellum is enlarged and this enlargement is most prominent in chelonians and lizards.
The cerebral hemispheres exhibit enlargement of palaeopallium and archipallium. The cells of these two parts of the pallium are shifted in reptiles towards the surface so as to show the beginning of cerebral cortex formation which is recognised in mammals.
In some reptiles a new structure, called neopallium, has appeared between the archipallium and palaeopallium. This new structure is absent in Sphenodon. The diencephalon houses several organs of special interest.
These organs include the large hypophysis and the parietal organ. The hypophysis is differentiated into an anterior and a posterior lobe. The parietal organ extends to the dorsal surface of the skull and forms the pineal or parietal eye. The parietal eye is well developed in Sphenodon. The optic lobe is somewhat reduced.
Eye:
The eyes of reptiles are variable in structure and adaptation. Among the reptiles the eyes of lizards are best developed. The eyes of snakes are greatly modified because of their nocturnal and burrowing habit.
On the cornea of some reptiles, a transparent covering formed by the fusion of the eyelids is present. The eyes of crocodiles have undergone modification due to nocturnal and aquatic adaptation. The eyes of turtles and Sphenodon are primitive in structure than that of lizards.
Ear:
The tympanum, when present, lies at the back of the jaws. In the inner ear, lagena has become longer. It has got on its inner wall a group of sense cells, called basilar papillae, which form the centre for auditory appreciation.
All the organs of the inner ear are housed in a perilymph cistern. This cistern has been formed by the tubular extension of the perilymph system. It has been shown that the ears of certain tortoises are sensitive to sound wave, a range of 110 cycles per second. The sense of hearing is best developed in crocodiles and lizards amongst the reptiles.
Other Sense Organs:
Sense organs for touch, pain and temperature are situated in the skin of reptiles. Organ of Jacobson is situated on the roof of the mouth and close to the internal nares. It is chemoreceptor in nature. Sensory epithelium is restricted on the dorsal part of the nasal chamber. Beginning of the turbinals is witnessed in the outgrowths from the wall of the dorsal part of the nasal chamber.
14. Excretory System of Reptiles:
The functional excretory organs are the metanephric kidneys. The shape of the kidneys in different reptiles is variable and the shape corresponds to the shape of the body cavity. In lizards and crocodiles the kidneys are much elongated and are housed in the posterior part of the body cavity.
In snakes the kidneys are long, slender and the position of the kidneys in the body cavity is asymmetrical, i.e., one kidney is located above its counterpart on the other side. In turtles, the kidneys are more compact than in other groups. The posterior ends of the kidney show a tendency of fusion in many lizards. The glomeruli inside the kidneys are reduced in reptiles.
The excretory ducts or ureters discharge their contents directly and separately into the cloaca. In male lizards the ureters unite with the vasa deferentia and open jointly in the cloaca, In turtles and many lizards, an urinary bladder is present. The ureters open in these cases into the urinary bladder. The urinary bladder is allantoic in origin.
In terrestrial reptiles the final product is excreted as uric acid. The final excretory product is a chalky white mass of urates and it is almost dry. The dryness is due to reabsorption of water in the urodaeum.
In lizards and snakes, 80-98% of the excretory product is uric acid. In aquatic turtles the excretory product contains little of uric acid and much of urea. In crocodiles and aquatic snakes up to 75% of the excretory product comprises of ammonia.
15. Reproductive System of Reptiles:
In reptiles the sexes are separate. Sexual differences are not easy to find out from external appearance. In some forms sexual colouration marks off the males from the females.
The general tendency exhibited by the reptiles in their reproductive system is that excretory and reproductive systems have become separated excepting their common opening into the cloaca. Because of the shape of the body the disposition of the left and right gonads in both males and females has become asymmetrical in some forms and specially in snakes.
Fertilization is internal in reptiles. All modern male reptiles (excepting Sphenodon) possess copulatory organs. These intromittent organs are derived from the wall of the urodaeum of the cloaca.
In crocodiles and tortoises, there is a single penis which is median in position but in lizards and snakes there is a pair of these structures laterally placed in the cloaca. They are called hemipenis. Table 39 shows the distinction of clasper and hemipenis. Out of these two, one is usually inserted at a time. Muscular action and vascular engorgement cause erection.
Oviparous reptiles lay their eggs on land. For physical support and protection against dessication, the eggs are provided with shell secreted by shell glands in the wall of the oviduct. For adequate supply of food during embryonic development, a large quantity of yolk is present inside the eggs.
Special embryonic membranes, known as amnion and allantois, look after gaseous exchange and storage of waste product. Egg albumen or egg white is present to a considerable amount in crocodiles and tortoises but in snakes and lizards the quantity of albumen present is very poor.
Many snakes and lizards are ovoviviparous. In forms like Lacerta vivipara, Anguis fragilis and Vipera berus, the eggs are retained within the oviduct till the youngs are ready or nearly ready to hatch. The eggs of ovoviviparous reptiles have the shell reduced to a membrane.
In sea snakes and certain lizards a placenta developed from the chorioallantois or the yolk-sac or both. The placenta serves for gaseous exchange and transfer of water. It is believed that in some advanced forms, the placenta provides a means for food supply.
To come out from the egg shells young reptiles have egg tooth. In Chelonia, Sphenodon and Crocodiles, the egg tooth is present on the snout. In Squamata, a true egg tooth is present which projects from the front of the upper jaw. Egg tooth is rudimentary in ovoviviparous forms.