Form and function ( reptile )
Tagged: animal, animals, bird, cat, dog, fish, insect, mammalExternal covering
The external covering of reptiles is characteristically dry. It bears few glands or none at all and differs in this respect from the skin of amphibians and mammals. The so-called malpighian layer of the epidermis secretes the outer layer, which is tough and horny. Bony plates develop in the dermis, which lies just below the epidermis. The arrangement of scales is usually characteristic for each species.
Internal features
Skeletal system and dentition
The skeleton of reptiles fits the general pattern of vertebrates. They have a bony skull, a long vertebral column that encloses the spinal nerve cord, ribs that form a bony basket around the viscera, and a framework of limbs. Each group of reptiles developed its own particular variations on this major pattern in accord with the general adaptive trends of the group. Snakes, for example, have lost the limb bones, although a few retain vestiges of the hindlimb. The limbs of several types of marine reptiles became modified into fins or flippers with obvious functional significance. In groups such as the extinct ichthyosaurs and plesiosaurs, the bones of the limbs, no longer supporting the weight of the body against the pull of gravity, became much shortened. At the same time the bones that in other reptiles composed the digits multiplied in number, forming a long flipper. Groups of reptiles whose modes of life came to depend heavily on passive defense also developed specializations of the skeleton. The bony and horny shell of turtles and rows of bony plates on the back of ankylosaurs (Cretaceous dinosaurs) are cases in point. The skulls of the several subclasses and orders vary in the ways mentioned below. In addition to differences in openings on the side of the skull and in general shape and size, the most significant variations in reptilian skulls are those affecting movements within the skull. Reptilian skulls as a group differ from those of early amphibians, the vertebrates from which reptiles arose, in lacking on otic notch (an indentation at the rear of the skull) and several small bones at the rear of the skull roof. The skulls of modern reptiles are sharply set off from those of mammals in many ways, but the clearest differences are in the lower jaw and adjacent regions. Reptiles have a number of bones in the lower jaw, only one of which, the dentary, bears teeth. Behind the dentary a small bone, the articular, forms a joint with the quadrate bone near the rear of the skull. In mammals the lower jaw consists of a single bone, the dentary, and the articular and quadrate have become part of the chain of little bones in the middle ear. An almost complete transition between these two very different arrangements is known from fossils of mammal-like reptiles (order Therapsida). The dentition of most reptiles shows little specialization along the tooth row. A division into distinctive bladelike incisors, tusklike canines, and flat-crowned molars, such as characterize mammals, does not occur in reptiles. Instead, the entire tooth row usually consists of long conical teeth. Venomous snakes have one or several hollow or grooved fangs, but they have the same shape as most snake teeth. The principal differences between species lie in the number, length, and position of the teeth. Crocodilians among the living forms and dinosaurs among the extinct forms have but a single upper and a single lower tooth row. Snakes and many extinct reptilian groups have teeth on the palatal bones (vomer, palatine, pterygoid) and on the bones of the upper jaw (premaxilla, maxilla); only one row of teeth is present on the lower jaw. Lizards have conical or bladelike bicuspid or tricuspid teeth. Some species have conical teeth at the front of the jaws and cuspid teeth toward the rear, but the latter are not comparable to the molars of mammals in either form (they are not flat-crowned) or function (they do not grind food). Turtles, except for the earliest extinct species, lack teeth, having instead upper and lower horny plates that serve to bite off chunks of food. The teeth of reptiles are also less specialized in function than are mammalian teeth. The larger carnivorous reptiles are equipped only to tear off or bite off large pieces of their prey and to bolt them without chewing. Insectivorous lizards (the majority of lizards) usually crack the exoskeleton of their insect prey, and then swallow the prey without grinding it up. Snakes simply swallow their prey whole without any mechanical reduction.
Skull and joint structures
Many reptiles developed joints (in addition to the hinge for the lower jaw) within the skull, permitting at least slight movement of one part relative to others. The capacity for such movement within the skull, called kinesis, enables an animal to increase the gape of the mouth and thus is an adaptation for swallowing large objects. Apparently some of the large carnivorous theropod dinosaurs (e.g., Allosaurus) had a joint between the frontal and parietal bones in the roof of the skull. All reptiles of the subclass Lepidosauria (lizards, snakes, rhynchocephalians, and the extinct eosuchians) have had kinetic skulls, but they differ from the dinosaurs in having the joint on the floor of the skull at the juncture of basisphenoid and pterygoid bones. The skulls of the lepidosaurians became increasingly kinetic as new groups evolved. The Rhynchocephalia (which include the living tuatara) and their antecedents, the Eosuchia, had only the basisphenoidal–pterygoidal joint. The lizards lost the lower temporal bar, thereby freeing the quadrate bone and allowing greater movement to the lower jaw, which is hinged to the quadrate. Finally, in the snakes, this trend culminates in the most kinetic skull among the vertebrates a skull having the ancestral basisphenoidal–pterygoidal joint, a highly mobile quadrate (which gives even greater mobility to the lower jaw), upper jaws capable of rotating on their longitudinal axes and of moving forward and backward, and often a hinge on the roof of the skull between the nasal and frontal bones that allows the snout to be raised slightly. In short, the only part of a snake’s skull incapable of movement is the braincase.
Nervous system
As in all vertebrates, the nervous system of reptiles consists of a brain, a spinal nerve cord, nerves running from the brain or spinal cord, and sense organs. Reptiles have small brains compared with mammals. The most important difference between the brains of these two vertebrate groups lies in the size of the cerebral hemispheres, the principal associative centres of the brain. In mammals these hemispheres make up the bulk of the brain and, when viewed from above, almost hide the rest of the brain. In reptiles the relative and absolute size of the cerebral hemispheres is much smaller. The brain of snakes and alligators forms less than 1â„1,500 of the total body weight, whereas, in mammals such as squirrels and cats, the brain accounts for about 1/100 of the body weight. A stegosaur (Stegosauria), roughly the size of an elephant, had a braincase no larger than that of a 2.4-metre (8-foot) crocodile, about large enough to contain a brain the size of a large walnut.
Circulatory system
Modern reptiles do not have the capacity for rapid sustained activity found in birds and mammals. It is generally accepted that this lower capacity is related to differences in the circulatory and respiratory systems. Before the origin of lungs, the vertebrate circulatory system had a single circuit: in the fishes, blood flows from heart to gills to body and back to the heart. The heart consists of four chambers arranged in a linear sequence. With the evolution of lungs in amphibians, a new and apparently more efficient circulatory system evolved. Two chambers of the heart, the atrium (or auricle) and ventricle, became increasingly important, and the beginnings of a double circulation appeared. An early stage in this evolution can be seen in amphibians today, where one of the main arteries from the heart (the pulmonary artery) goes directly to the lungs, whereas the others (the systemic arteries) carry blood to the general body. The blood is aerated in the lungs and carried back to the atrium of the heart. From the left side of the atrium, which is at least partially divided for the first time, the aerated blood is pumped into the ventricle and there mixes with the nonaerated blood from the body that was returned to the heart via the right half of the atrium. Then the cycle begins again. One of the features of the amphibian system is that the blood leaving the heart for the body is only partially aerated; part of it is the deoxygenated blood returned from the body. All groups of modern reptiles have a completely divided atrium; it is safe to assume, therefore, that this was true of most, if not all, extinct reptiles. In reptiles, the ventricle for the first time becomes at least partially divided in the four major living groups. When the two atria of a lizard’s heart contract, the two streams of blood (aerated blood from the lungs in the left atrium and nonaerated blood from the body in the right atrium) flow into the left chamber of the ventricle. As pressure builds up in that chamber, the nonaerated blood is forced through the gap in the partition into the right chamber of the ventricle. Then, when the ventricle contracts, nonaerated blood is pumped into the pulmonary artery and thence to the lungs, while aerated blood is pumped into the systemic arteries (the aortas) and so to the body. In snakes all three arterial trunks come out of the chamber of the ventricle that receives the nonaerated blood of the right atrium. During ventricular contraction, a muscular ridge forms a partition that guides the nonaerated blood into the pulmonary artery, while the aerated blood received by the other chamber of the ventricle is forced through the opening in the ventricular septum and out through the aortas. In crocodiles the ventricular septum is complete, but the two aortas come out of different ventricular chambers. A semilunar valve at the entrance to the left aorta prevents nonaerated blood in the right ventricle from flowing into the aorta. Instead, part of the aerated blood from the left ventricular chamber pumped into the right aorta flows into the left by way of an opening. The ventricle of the turtle is not perfectly divided, and some slight mixing of aerated and nonaerated blood takes place. Despite the peculiar and complex circulation, a double system has been achieved by lizards, snakes, and crocodilians. Tests of the blood in the various chambers and arteries have shown that the oxygen content in both systemic aortas is as high as that of the blood just received by the left atrium from the lungs and is much higher than that of the blood in the pulmonary artery. Except for the turtles, limitation of activity in reptiles cannot be explained on the basis of imperfect heart circulation. An explanation may lie in the chemistry of the blood. Apparently, the blood of reptiles has less hemoglobin and thus can carry less oxygen that that of mammals.
Respiratory system
The form of the lungs and the methods of irrigating them may also influence activity by affecting the efficiency of respiration. In snakes the lungs are simple saclike structures having small pockets, or alveoli, in the walls. In the lungs of many lizards and turtles and of all crocodilians the surface area is increased by the development of partitions that, in turn, have alveoli. Because exchange of respiratory gases takes place across surfaces, an increase of the ratio of surface area to volume leads to an increase in respiratory efficiency. In this regard, the lungs of snakes are not so effective as those of crocodilians. The elaboration of the internal surface of lungs in reptiles is simple, however, compared with that reached by mammalian lungs with their enormous number of very fine alveoli. Most reptiles breathe by changing the volume of the body cavity. By contractions of the muscles moving the ribs, the volume of the body cavity is increased, creating a negative pressure, which is restored to atmospheric level by air rushing into the lungs. By contraction of body muscles, the volume of the body cavity is reduced, forcing air out of the lungs. This system applies to all modern reptiles except turtles, which, because of the fusion of the ribs with the rigid shell, are unable to breathe by this means; they do use the same mechanical principle of changing pressure in the body cavity, however. Contraction of two flank muscles enlarges the body cavity, causing inspiration. Contraction of two other muscles, coincident with relaxation of the first two, forces the viscera upward against the lungs, causing exhalation. The rate of respiration, like so many physiological activities of reptiles, is highly variable, depending in part upon the temperature and in part upon the emotional state of the animal.
Digestive and urogenital systems
The digestive system of modern reptiles is similar in general plan to that of all higher vertebrates. It includes the mouth and its salivary glands, esophagus, stomach, and intestine, ending in a cloaca. Of the few specializations of the reptilian digestive system, the evolution of one pair of salivary glands into poison glands in the venomous snakes is the most remarkable. During development the embryos of higher vertebrates (reptiles, birds, and mammals) use three separate sets of kidneys consecutively; these are arranged in longitudinal sequence in the body cavity. The first set, the pronephroi, are vestigial organs left over from the evolutionary past that soon degenerate and disappear without having had any function. The second set, the mesonephroi, are the functional kidneys of adult amphibians, but their only contribution to the lives of reptiles is in providing the duct (the wolffian duct) that forms a connection between the testis and the cloaca. The operational kidneys of reptiles, birds, and mammals are the last set, the metanephroi, which have separate ducts to the cloaca. The principal function of the kidney is the removal of nitrogenous wastes resulting from the oxidation of proteins. Vertebrates eliminate three kinds of nitrogenous wastes: ammonia, urea, and uric acid. Ammonia and urea are highly soluble in water; uric acid is not. Ammonia is highly poisonous, urea slightly so, and uric acid not at all. Among reptiles the form taken by the nitrogenous wastes is closely related to the habits and habitat of the animal. Aquatic reptiles tend to excrete a large proportion of these wastes as ammonia in solution. This method, involving a great loss of body water, is no problem for an alligator, which eliminates between 40 and 75 percent of its nitrogenous wastes as ammonia. Terrestrial reptiles, such as most snakes and lizards, which must conserve body water, convert their nitrogenous wastes to insoluble, harmless uric acid, which forms a more or less solid mass in the cloaca. In snakes and lizards these wastes are eliminated from the cloaca together with wastes from the digestive system. Prior to the evolution of the metanephric kidney, the products of the male gonad, the testis, travelled through the same duct with the nitrogenous wastes from the kidney. But with the appearance of the metanephros, the two systems became separated. The female reproductive system never shared a common tube with the kidney. Oviducts in all female vertebrates arise as separate tubes with openings usually near, but not connected to, the ovaries. The oviducts, like the wolffian ducts of the testes, open to the cloaca. Both ovaries and testes lie in the body cavity near the kidneys. With the evolution of the reptilian egg, internal fertilization became necessary. The males of all modern reptiles, with the exception of the tuatara, have copulatory organs. The structures vary from group to group, but all include erectile tissue as an important element of the operating mechanism, and all are protruded through the male’s cloaca into that of the female during copulation. Unlike the penis of turtles and crocodilians, the copulatory organ of lizards and snakes is paired, each unit being called a hemipenis. The hemipenes of lizards and snakes are elongated tubular structures lying in the tail. The penis of a crocodile or turtle is protruded through the cloacal opening wholly by means of a filling of blood space (sinuses) in the penis; protrusion of a lizard’s or snake’s hemipenis, however, is begun by a pair of propulsor muscles. Completion of the erection is brought about by blood filling the sinuses in the erectile tissue. Only one hemipenis is inserted into a female, but which one is a matter of chance. Unlike the penis of mammals, the copulatory organs of reptiles do not transport sperm through a tube. The ducts from the testes, as already mentioned, empty into the cloaca, and the sperm flow along a groove on the surface of the penis or hemipenis.
Sense organs
Sight
In general construction the eyes of reptiles are like those of other vertebrates. Accommodation for near vision in all living reptiles except snakes is accomplished by pressure being exerted on the lens by the surrounding muscular ring (ciliary body), which thus makes the lens more spherical. In snakes the same end is achieved by the lens being brought forward under pressure built up on the vitreous humour by contractions of muscles at the base of the iris. The pupil shape varies remarkably among living reptiles, from the round opening characteristic of all turtles and many diurnal lizards and snakes to the vertical slit of crocodilians and nocturnal snakes and the horizontal slits of a few tree snakes. Undoubtedly the most bizarre pupil shape is that of some geckos, in which the pupil contracts to form a series of pinholes, one above the other. The lower eyelid has the greater range of movement in most reptiles. In crocodilians the upper lid is more mobile. Snakes have no movable eyelids, their eyes being covered by a fixed transparent scale. The tuatara and all crocodilians have a third eyelid, the nictitating membrane, a transparent sheet that moves sideways across the eye from the inner corner, cleansing and moistening the cornea without shutting out the light. Visual acuity varies greatly among living reptiles, being poorest in the burrowing lizards and snakes (which often have very small eyes) and greatest in active diurnal species (which usually have large eyes). Judging by the size of the skull opening in which the eye is situated, similar variation existed among the extinct reptiles. Those that hunted active prey (e.g., the ichthyosaurs) had large eyes and presumably excellent vision; many herbivorous types (e.g., the horned dinosaur Triceratops) had relatively small eyes and weak vision. Colour vision has been demonstrated in few living reptiles.
Hearing
The power of hearing is variously developed among living reptiles. Crocodilians and most lizards hear reasonably well. Snakes and turtles are sensitive to low-frequency vibrations, thus they “hear†mostly earth-borne, rather than aerial, sound waves. The auditory apparatus in reptiles typically consists of a tympanum, a thin membrane located at the rear of the head; a small bone, the stapes, running between the tympanum and the skull in the tympanic cavity (the middle ear); the inner ear; and a eustachian tube connecting the middle ear with the mouth cavity. In reptiles that can hear, the tympanum vibrates in response to sound waves and transmits the vibrations to the stapes. The inner end of the stapes abuts against a small opening (the foramen ovale) to the cavity in the skull containing the inner ear. The inner ear consists of a series of hollow interconnected parts: the semicircular canals; the ovoidal or spheroidal chambers called the utriculus and sacculus; and the lagena, a small outgrowth of the sacculus. The tubes of the inner ear, suspended in a fluid called perilymph, contain another fluid, the endolymph. When the stapes is set in motion by the tympanum, it develops vibrations in the fluid of the inner ear; these vibrations activate cells in the lagena, the seat of the sense of hearing. The semicircular canals are concerned with equilibrium. Most lizards can hear; details of the acuity of hearing, however, are largely unknown. The majority have a tympanum, tympanic cavity, and eustachian tube. The tympanum, usually exposed at the surface of the head or at the end of a short open tube, may be covered by scales or may be absent. In general the last two conditions are characteristic of lizards that lead a more or less completely subterranean life and presumably do not hear airborne sounds. The middle ear of these burrowers is usually degenerate as well, often lacking the tympanic cavity and eustachian tube. Snakes have neither tympanum nor eustachian tube, and the stapes is attached to the quadrate bone on which the lower jaw swings. Snakes are obviously more sensitive to vibrations in the ground than to airborne sounds. A loud sound above a snake does not elicit any response provided the object making the sound does not move or, if it does, the movements are not seen by the snake. On the other hand, the same snake will raise its head slightly and flick its tongue in and out rapidly if the ground behind it is tapped or scratched. Snakes undoubtedly “hear†these vibrations by means of bone conduction. Sound waves travel more rapidly and strongly in solids than in the air and are probably transmitted first to the inner ear of snakes through the lower jaw, which is normally touching the ground, thence to the quadrate bone, and finally to the stapes. Burrowing lizards presumably hear ground vibrations in the same way. Crocodilians, all of which have an external ear consisting of a short tube closed by a strong valvular flap and ending at the tympanum, have rather keen hearing. The American alligator (Alligator mississippiensis) can hear sounds within a range of 50 to 4,000 cycles per second. The hearing of crocodilians is involved not only in detection of prey and enemies but also in their social behaviour, for males roar or bellow either to threaten other males or to attract females. Although turtles have well-developed middle ears and usually large tympana, their ability to hear airborne sounds is still an open question. Measurements of the impulses of the auditory nerve between the inner ear and the auditory centre of the brain show that the inner ear in several species of turtles is sensitive to airborne sounds in the range of 50 to 2,000 cycles per second, but this does not prove that the animals are aware of the sounds.
Chemoreception
Chemical-sensitive organs, used by many reptiles to find their prey, are located in the nose and in the roof of the mouth. Part of the lining of the nose consists of cells subserving the function of smell and corresponding to similar cells in other vertebrates. The second chemoreceptor is Jacobson’s organ, originally an outpocketing of the nasal sac in amphibians and remaining so in the tuatara and crocodilians. It has been lost by turtles. Jacobson’s organ is best developed in lizards and snakes, in which its connection with the nasal cavity has been closed and is replaced by an opening into the mouth. The nerve connecting Jacobson’s organ to the brain is a branch of the olfactory nerve. The use of Jacobson’s organ is most obvious in snakes. If a strong odour or vibration stimulates a snake, its tongue is flicked in and out rapidly. With each retraction the forked tip touches the opening of Jacobson’s organ in the roof of the mouth, transmitting any chemical fragments adhering to the tongue. In effect, Jacobson’s organ is a supplement to taste and is a short-range chemical receptor, as contrasted with the long-range testing of the true sense of smell located in the nasal tube. Some snakes (notably the large vipers) and lizards (especially skinks and burrowing species of other families) rely upon the olfactory tissue and Jacobson’s organ to locate food, almost to the exclusion of other senses. Other reptiles, such as certain diurnal lizards and crocodilians, appear not to use scent in searching for prey, though they may use their sense of smell for locating a mate. Some snakes, notably pit vipers, boas, and pythons, have special heat-sensitive organs on their heads as part of their food-detecting apparatus. Just below and behind the nostril of a pit viper is the pit that gives the group its common name. The lip scales of many pythons and boas have depressions (labial pits) that are analogous to the viper’s pit. The labial pits of pythons and boas are lined with skin thinner than that covering the rest of the head and are supplied with dense networks of blood capillaries and nerve fibres. The facial pit of the viper is relatively deeper than the boa’s labial pits and consists of two chambers separated by a thin membrane bearing a rich supply of fine blood vessels and nerves. In experiments using warm and cold covered electric light bulbs, pit vipers and pitted boas have been shown to detect temperature differences of less than 0.6° C (1.1° F). Since many pit vipers, pythons, and boas are nocturnal and feed largely on mammals and birds, the facial sense organs enable them to direct their strikes accurately in the dark, once their warm-blooded prey arrives within range. The approach of the prey to that point is probably detected by the chemical receptors either the nose, Jacobson’s organ, or both.
Thermal relationships
Reptiles are often described as being cold-blooded, which is not always true. Their body temperatures are not always low, but they have no internal mechanism for regulating body temperature and thus approximate closely the temperature of their surroundings. This condition is termed poikilothermy. Mammals and birds maintain their relatively high body temperatures at a fairly constant level by physiological means that are independent of the external environment, a condition called homoiothermy. When the body temperature of a dog or a man falls below the normal range, he begins to shiver, blood vessels in the skin contract, muscular activity generates heat, and the contraction of the superficial blood vessels, by reducing the volume of blood flow at the surface, reduces heat loss by radiation. By contrast, a reptile, when its body temperature falls below the optimum, must move to some portion of the environment having a higher temperature; in less than optimal temperatures, its activity drops, its movements become sluggish, its heartbeat slows, and its rate of breathing drops. In short, it becomes incapable of the normal activities required for growth, reproduction, and survival. Mammals and birds have some physiological means of cooling their bodies (e.g., panting and sweating, expansion of superficial blood vessels), but a reptile must ordinarily move away from a spot in which the temperature is too high or it will perish very quickly. Some reptiles also pant, but most of their temperature accommodations are behavioral (e.g., orienting to sun or wind, raising body from the ground). Each group of reptiles has its own characteristic thermal range. One genus of lizards, for example, may require temperatures of 29°–32° C (84°–90° F) for maximum efficiency, and another may require 24°–27° C (75°–81° F). As a result of such physiological differences, lizards of the two groups will be active at different times of the day or occupy slightly different habitats. In general the normal activity temperatures of reptiles are lower than those of most mammals; however, a few sun-loving (heliothermic) lizards (e.g., the greater earless lizard, Holbrookia texana, of southwestern United States) have average activity temperatures above 38° C (100° F), several degrees higher than the average human body temperature. Such high temperatures are exceptional, and the majority of lizards have normal activity temperatures in the 27°–35° C (81°–95° F) range.
Comments
Got something to say?