General Characteristics of Class Amphibia:
Amphibians have moist skin that helps them in gaseous exchange in supplement to lungs. They have no scales. Their skeleton is bony. There are 2 pairs of limbs which end in 5 fingers. These are known as pendactyle limbs. Visceral clefts are present in aquatic larvae (tadpole) only. Adults are terrestrial and contain lungs. There is metamorphosis from larvae to adult in life cycle. They do not have external ear. They lay eggs that are fertilized externally. Adults come back to water for reproduction.
For animals which are mainly aquatic to live in land, there should structural and physiological variation. Amphibians are vertebrates which live part of the life in water and part on land. They pass through the fish-like phase in their early life. Reptiles are first group of vertebrates which are completely land animals. They grown-up to adulthood from time egg is laid on land. Birds and mammals are further adaptations or alterations of reptiles. Amphibian absorbs oxygen through skin and badly developed lungs. They are of interest in respect of colonization of land by vertebrates as in life cycle of every amphibian they begin in water and end on land. Eggs are laid in water. Larvae (tadpole) of frogs lack paired fins, whereas larvae of salamanders contain legs in place of fins. Except for these, aquatic larvae of amphibians are essentially fishlike. They have similar circulatory and respiratory systems such as fish. They swim like fish. There are 2 chambered heart and 4 pairs of gills. Egg of amphibians undergoes metamorphosis in more terrestrial animals. In procedure of metamorphosis they suffer variations associating to movement towards terrestrial life. Tadpole of frog is vegetarian. Because of horny skin teeth, it feeds on vegetation, and swims with help of fish like tail. When it reaches the definite size, it changes, develops hind and fore limbs, reabsorbs tail, guts degenerate and lungs enlarge. Circulatory system also is changed in relation to exchange of gases. Animal is compelled to leave water if it is not to drown.
General Characteristics of Class Reptilia:
Reptiles contain dry scaly skins. Scales are horny, have bony skeleton. There are 2 pairs of pentadactye limbs. Their visceral clefts never grow gills. They do not have external ears. Their fertilized yolky eggs are kept by reptile until hatching. Fertilization is internal. Eggs have the leathery skin. They have beak grown in place of teeth.
Reptiles are the example of vertebrates of 4 legged (tetrapod) kinds. Lizard has been explained as being completely adapted to life on land from starting of life cycle to adulthood. Reptiles have kept streamline shape. Its body is enclosed with horny scales that don't permit water to go through through it. Body is thus resistant to water loss in dry water. Four legs are very strong and joined beneath body so that it provides support to body and forces animal forwards as it moves. Lungs and circulation are adapted for aerial life. Eggs are kinds which are laid on land and can expand without passing the aquatic larval stage.
Adaptive Features and Evolutionary Development:
1) Body and Locomotion:
Body of lizard comprises of head, trunk and tail. Head is visibly raised from ground on the well grown neck. The body maintains streamlined, tapering towards laid. Head slightly blunt at anterior end, and body widening onto middle. 2 limbs are joined to pectoral and pelvic girdles and are placed under body. Girdles are formed like bracers. With limbs they give effective support to weight of animal. Tails appear to serve primarily for balancing animal and preventing it from toppling over its nose. Limbs themselves should be attached to firm frames that in turn articulate with vertebral column that should be stronger, bonier and more directly articulated and less flexible.
In preparation for locomotion body is raised by limb and girdle muscles that form lever along with skeleton of limbs. Limb arrangement is strongly connected to fish patters. Parts of limb nearer body are less than those farther away. Gradual increase in parts gives fan-like arrangement. Further distal are many bones of wrist and ankle. Then there are 5 rows of slender bones in hand or foot which end with 5 fingers or toes. This is ancestral pentadactye limb from which those of vertebrate ones have developed.
2) Respiration and Blood Circulation:
Respiration is the process which is seriously influenced by movement out of water. Reptiles depend completely on land. Ribs and rib muscles bring in air by increasing chest or thorax. Arteries now lead to lungs, head and dorsal aorta. In amphibians and reptiles blood runs in 3 arterial paths; one splits to left and right to provide lungs; another to left and right part of head (carotid arteries) and third splits left and right then join up above digestive tube to form dorsal aorta. This last one is systematic arch. It matches to pair of gill arteries without gill capillary system. This is like arrangement in fish gill arteries that is in parts covered up and in few places enlarged. It is like the system -readjusted to another. This tadpole swims similar to fish and has complete gills with distinctive gill arteries. Further, as the result of transform from gill to lungs, heart is influenced; in fish and tadpole (or immature aquatic stage of amphibians) heart has 2 chambers. It pumps blood that gets oxygenated through gill capillary vessels. Blood then move under pressure to body tissues. The 2 chambered hearts here manages only venous blood (this is deoxygenated blood). In this way it gains vigor of pumping and vitality of oxygen. This is achieved by device that virtually divides heart into 2 (though separation is not complete) chambers. Right half auricular chamber and right half of ventricle. It pumps blood forward to lungs. Here blood obtains oxygenated and returns directly to other half of heart. This other half pumps oxygenated blood forward to head and body. Ventricle permits some mixed material and venous blood to pass on to tissues, in both frog and lizard, as partition is not complete.
3) Water Conservation:
The main success of land animals arises from the capacity to conserve water. Horny protein material of skin of terrestrial vertebrates is composed of keratin. Keratin is utilized to form overlying scales of reptiles. It is hard, dry and unreceptive to water. It is light weight protective layer that prevents water loss over body surface. Only source of water loss is through lungs in respiration. Excretory organs also assist in water conservation. Fish kidneys are effective water eliminators as they should carry on to send out water which constantly enters animal. This type of kidney will not assist reptile. Therefore in reptile, most of water which carries wastes excretion is reabsorbed from drainage tubules. Water is reabsorbed from dilute nitrogenous wastes in form of ammonia to form uric acid that is passed out almost in solid form. By two processes, body surface control of water loss and activity of kidneys, body tissues keep their continuous liquid medium.
4) Senses and Brains:
Water pressure and vibration sensing organs comprising head sense organs and lateral line vanish in time of metamorphosis in amphibian. So also are sensory structures associated to gills as air is now drawn into nose through olfactory organs and in throat by internal nasal openings. Tetrapod characteristics had to expand to bring chemicals in air in contact with living of organs of smell. Eyes also had to be adjusted to function out of water. Due to difference in refractive index of air, cornea had to become curved and liable for projecting mirage out retina. As it is no more in water eyes needed to be kept moist and protected with eyelids, inchitating membrane, and glands. Organ of hearing on land comprises of organ itself and resources of transmitting air borne waves to it.
The Amniotic Eggs:
The amphibians that continue to set the eggs in water are not true land animals. Reptiles have evolved the kind of egg which can place on land. It is this kind that birds and mammal lay as well. No aquatic stage is required in development of the egg that is known as amniotic egg. Emergence of amniotic egg represents the important invention in history of life. To be able to expand out of water, egg requires given:
i) The shell for protection against desiccation and to support semi liquid egg cell against pull of gravity.
ii) Yolk within cell, adequate to permit embryo to grow large sufficient to omit larval stages and go on directly to become small forms of adult types
iii) Water for growing tissues within
iv) The store for toxic materials made.
All these conditions are met in amniotic eggs of true only need for water by terrestrial animals is for drinking. It though has its constrains on growth and fertilization. With shell, it means that egg should be fertilized prior to it is enclosed in shell. This means sperm should be transferred into female before egg is laid. Development of egg in shell is also important.
i) The yolk gives food for growing embryo. It is not engaged in cell division. It is surrounded in yolk sac.
ii) Liquid waste material made by growing embryo is stored in allantoic sac. Both yolk and allantoic sac are enclosed by blood circulation that connects them to activity dividing embryo. They supply food (yolk) and provide means for excretion (allantoic fluid).
iii) Amnion forms the pair of folds that cover embryo and surround amniotic fluid so that even when placed on dry land, it grows within aquatic environment. It is from this layer egg got its name.
iv) The chorion, is the outermost layer of cells. It encloses embryo and embryonic membranes. Amnion, chonon yolk sac and allantois are all composite structures comprising of 2 layers of tissue, either ectoderm and mesoderm, as in chonon and amnion, or endoderm, and mesoderm, since in yolk sac and allantous.
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