Page:The New International Encyclopædia 1st ed. v. 20.djvu/131

VEKTEBRATA. 97 VEKTEBRATA. cally of an axial series of parts replacing the chorda; of a box (cranium), inchising the brain; of a series of arches (neiinil arclu's) jiriitccting the nerve-tube; and of supports to the body-wall (ribs). The heart is ventral to the alimentary tract, which begins in a ventrally placed mouth and ends in a ventral anus, placed near the base of the tail. Vertebrates are the only group of animals in which the body nuiy be divided into head, trunl<, and tail, the tail being postanal. Tliiy alone are characterized by having typically, except as lost by degeneration, four limbs. They have an outer skin that consists of nion' than a single layer of cells. They liave an internal skeleton that is essentially living, containing active cells within it. Each pair of lind)s, when present, is supported by a bonj- girdle, which may in extreme cases be rudimentary. The skull, ex- cept in Cyclostomi, consists of cranium and lower jaw. The jaws are typically provided with teeth, but these are lost in turtles and birds. The cen- tral nervous .system consists of a nerve-tube (spinal cord) and brain, with which latter the main sense-organs are connected. The brain is the special seat of sense-perception, of voluntary motor-impulses, and of the higher intellectual faculties. From brain and spinal cord a meta- merically repeated series of nerves runs off to the skin, viscera, and muscles. The sense-organs con- sist of an olfactory pit usually paired ; paired camera eyes, with lens ; a pair of auditory or- gans (pits or closed cavities) usually comprising three semicircular canals. In addition to the cerebrospinal system is a visceral system (the sympathetic). The digestive tract is a tube sep- arated from the body-wall by the peritoneum- lined body-cavity. The digestive tube is divided into oesophagus and stomach, the small intestine w'ith liver and pancreas, and the large intestine. (See Alimentary System.) Respiratory organs are typically present, either as gills in the aquatic vertebrates or as lungs. In some sala- manders, however, respiration is by the throat and entire skin. The gills (q.v.) are thin plates typically placed in two rows on the gill-arches between the slits. The lungs, which arise, ap- parently, as swimming bladders in fishes, come to lie ventral and to have a purely respiratory function in land vertebrates. The circulatory system (q.v.) consists of a set of vessels almost wholly cut off from the body-cavity, and carry red blood, due to the red blood-corpuscles. This fluid is propelled by a heart that consists essen- tially of two parts, a thin-walled auricle that receives blood entering from the trunk behind, and a thick-walled muscular ventricle that forces the blood out forward, to the respiratory organs, head, and trunk. In addition to the blood-vessels are the lymph-vessels, spaces incompletely cut off from the body-cavity, and carrying cliiefly white blood-corpuscles and special nutriment to the blood. Special excretory organs (see Kxcretory System) are present, essentially consisting of paired, metaraerically repeated tubules into which blood-vessels pour their waste material. The tubules empty into a common paired duct, which also may serve, in the male, to convey the sexual products to the exterior. Reproduction is always sexual arid the sexes are usually separate. See Reproductive System; JIuscuLr System. Development usually .starts with a yolk-laden egg tluit cleaves incorupletely. The embryo first appears as a disk or l)lastoderm lying on the yolk. At the hinder edge of the growing disk some cells are pushed in to form the alimentary tract. A pair of parallel folds rise up along the back to form the neural cerebral tube. From the upper right and left angles of the cavity of the alimentary tract inetanieiically re|)eated pairs of pouches are cut off to form the mesodermal pouches from whose walls the muscles and lining of the body-cavity arise. The notochord develops first as a continuous groove, and finally as a rod, between the mesodermal mas.ses. By the time the germ-disk has surrounded the yolk the body- wall is eom]jlelely formed. The yolk lies at the bottom of I lie gut and forms a huge lump there, which gradually becomes absorbed and the animal assumes the typical cylindrical form. See Embryology. The vertebrates form the section Craniata of the phylum Chordata, and embrace six classes, namely: the cyelostomes, fishes, amphibians, reptiles, birds, and mammals. See Classifica- tion OF Animals : and the names of these vari- ous classes, and of their subordinate groups. mode of evolution of the vertebrata. At a very early period in the history of vertebrates, the internal rod or backbone, at first cartilaginous, became ossified and segmented into vertebrie, and four appendages developed which became fins, legs, or wings, for progression in water, on land, or in the air, respectively. Along the upper side of the backbone lay the spinal cord, holding the cells and nerves which co- ordinated the action of the different parts of the organism, and the anterior end of this cord early specialized into a brain. The sense-organs and the mouth were grouped around the brain, and the bony structures which developed to protect them and facilitate their action constituted the skull, which became the most characteristic and highly differentiated part of the animal. Many vertebrates developed various protective bony structures over the surface of the body as well as the head. The higher specialization of an animal, its more exact adaptation to a particular mode of existence, are an advantage to it so long as the conditions of its environment remain the same. But if the conditions change, the highly special- ized type is at a disadvantage, as it cannot so well adapt itself to a new environment as its more generalized rivals. It is, therefore, likely to become extinct and be replaced by the less specialized forms, which in turn become highly specialized in adaptation to the new conditions. The many and great geological changes which have taken place involve many and complex changes in the conditions of life, both of marine and land animals, and changes in each group of organisms involve a widening circle of change in all the other groups dependent on it. But in the more complex modes of life a higher intelli- gence and a better mechanical adaptation to the more general conditions of the environment have been at all times of advantage. We observe, therefore, In the history of vertebrates through geological time, a general continuous progress in intelligence and in the broader lines of adapta- tion, together with a series of successive speciali- zations in the details of their construction. The greater size of an animal is of advantage to