The state or period of infancy. The quality of being a baby; the personality of an infant.
Saltatorial locomotion provides a rapid means of travel, but requires enormous development of hind limb muscles. The large muscular tail of the kangaroo is used for balance.
The bodies of gliding and flying vertebrates tend to be shortened and relatively rigid, although the neck is quite long in many birds see Fig. The skin of vertebrates is composed of an outer layer known as epidermis and an inner layer known as dermis and serves as the boundary between the animal and its environment.
Among vertebrates, skin collectively functions in protection, temperature regulation, storage of calcium, synthesis of vitamin D, maintenance of a suitable water and electrolyte balance, excretion, gas exchange, defense against invasion by microorganisms, reception of sensory stimuli, and production of pheromones chemical substances released by one organism that influence the behavior or physiological processes of another organism.
Significant changes, particularly in the epidermis, occurred as vertebrates adapted to life in water and later to the new life on land. The entire epidermis of fishes consists of living cells. Numerous epidermal glands secrete a mucus coating that retards the passage of water through the skin, resists the entrance of foreign organisms and compounds, and reduces friction as the fish moves through water.
The protective function of the skin is augmented by dermal scales in most fishes. The move to land brought a subdivision of the epidermis into an inner layer of living cells, called the stratum germinativum, and an outer layer of dead cornified cells, called the stratum corneum.
In some vertebrates, an additional two to three layers may be present between the stratum germinativum and stratum corneum. The stratum corneum is thin in amphibians, but relatively thick in the more terrestrial lizards, snakes, crocodilians, birds, and mammals, where it serves to retard water loss through the skin.
Terrestrial vertebrates developed various accessory structures to their integument such as scales, feathers, and hair as adaptations to life on land. Many ancient amphibians were well covered with scales, but dermal scales occur in modern amphibians only in the tropical, legless, burrowing caecilians, in which they are rudimentary or degenerate vestigial and embedded in the dermis.
The epidermal scales of turtles, lizards, snakes, and crocodilians serve in part to reduce water loss through the skin, serve as protection from aggressors, and in some cases snakesaid in locomotion.
The evolution of endothermy in birds and mammals is associated with epidermal insulation that arose with the development of feathers and hair, respectively. Feathers are modified reptilian scales that provide an insulative and contouring cover for the body; they also form the flight surfaces of the wings and tail.
Unlike feathers, mammalian hair is an evolutionarily unique epidermal structure that serves primarily for protection and insulation. Some land vertebrates have epidermal scales underlain by bony plates to form a body armor. For example, turtles have been especially successful with this type of integumental structure.
Among mammals, armadillos Dasypus and pangolins Manis have similar body armor Fig. Armadillos, which can run rapidly and burrow into loose soil with lightninglike speed, are also good swimmers.
The neural arch is dorsal to the centrum and encloses the spinal cord. The hemal arch, when present, is ventral to the centrum and encloses blood vessels. Cornified keratinized epidermal tissue has been modified into various adaptive structures in terrestrial vertebrates, including scales, feathers, and hair.
The tips of the digits are protected by this material in the form of claws, nails, or hooves. The horny beaks of various extinct diapsids, living turtles, and birds have the same origin. The central element of the skeleton is the vertebral column, which is made up of individual vertebrae.
There is no typical vertebra; a composite is shown in Fig. Each vertebra consists of a main element, the centrum, and various processes. An Overview The vertebral column of fish consists of trunk and caudal vertebrae, whereas in tetrapods four-legged vertebratesthe vertebral column is differentiated into a neck cervical region, trunk region, sacral region, and tail caudal region.
In some lizards and in birds and mammals, the trunk is divided into a rib-bearing thoracic region and a ribless lumbar region.
Two or more sacral vertebrae often are fused in tetrapods for better support of body weight through the attached pelvic girdle; this is carried to an extreme in birds with the fusion of lumbars, sacrals, and some caudals. Neural arches project dorsally to enclose and protect the nerve cord, and in fishes, hemal arches project ventrally to enclose the caudal artery and vein.
The skull supports and protects the brain and the major special sense organs. In hagfish, lampreys, and cartilaginous fish, the skull is cartilaginous and is known as the chondrocranium, but in other vertebrates, bones of dermal origin invade the chondrocranium and tend to progressively obscure it.
It is believed that primitive vertebrates had seven gill arches and that elements of the most anterior gill arch evolved into the vertebrate jaw, which was braced by elements of the second gill arch see discussion in Chapter 5.
As vertebrates continued to evolve, dermal plates enclosed the old cartilaginous jaw and eventually replaced it.National Lampoon's Christmas vacation [videorecording] / Warner Bros.
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