REPTILES 275 being effected by deglutition. Only a small portion of the blood is sent to the lungs, and this is feebly oxygenated, as the respiration is performed slowly and the lung is of loose texture and small capacity ; hence a low de- gree of animal heat, languid movements, and a slow performance of the nutritive functions. They have no true epiglottis and no proper voice, though some emit a hissing sound (as the ophidians) formed in the mouth. The heart has four cavities, but the ventricles com- municate, except in the crocodilians, where an admixture of the arterial and venous bloods takes place in the great vessels ; there is, therefore, a partial circulation independent of respiration, enabling them to remain long un- der water and in irrespirable gases. The lym- phatic system is greatly developed, having I'egular pulsating organs' or lymphatic hearts for the propulsion of their fluid. Reptiles eat and drink comparatively little, and are able to go a long time without food ; not hav- ing movable and fleshy lips, they cannot per- form the act of suction, as was once popularly believed of serpents ; the mouth is generally large, and the lower jaw articulated by a dis- tinct bone, the homologue of the os quadratum of birds. The tongue is generally free, and the oes'ophagus very wide and distensible to accommodate large prey ; the intestine is short and straight in proportion to the carnivorous disposition, being longest in the herbivorous chelonians and shortest in the snakes ; there is a certain division into small and large in- testine, though the latter in most is properly the rectum ; the alimentary canal opens below into a cloaca, or cavity common to the diges- tive, urinary, and reproductive organs, as in birds ; all the nutritive elements are extracted from the food, the indigestible matters being ejected in a mass at long intervals ; the vent is transverse in snakes and lizards, but longitudi- nal in chelonians and crocodiles, corresponding to remarkable differences in the male external reproductive organs, these in the former being double and placed in a cavity behind the anus, and in the latter single and within the cloaca. Salivary glands, which are absent in fishes and batrachians, are present in reptiles ; the liver is always present and large, receiving much venous blood, especially that from the posterior part of the body ; the gall bladder is common- ly found, though small ; the spleen is generally very small, removed from the liver and stomach, rounded, and deep red; the pancreas is con- stant, often large at the beginning of the intes- tine, and of various forms ; the kidneys are situated along the spine, showing no distinc- tion of cortical and medullary portions ; the ureters open into the cloaca, and the urine is a whitish mass, more or less hard, containing salts of lime and ammonia; the supra-renal capsules are usually present, small, and often remote from the kidneys ; there are one poste- rior and two anterior venje cavro. The power of reproducing lost parts is less than in batra- chians, and is noticed especially in the tails of certain lizards and serpents. In this class there is no durable union of the sexes as hi birds and mammals, and nothing which exerts any in- fluence on the social condition of the individ- uals ; after the instinctive act of reproduction they separate and become perfect strangers. Most are oviparous, leaving their eggs to be hatched by the heat of the sun, and the young when born are able to provide for themselves and are generally indifferent to the mother; the female rarely makes a nest, but deposits her eggs in a safe, warm, and dry place; croc- odiles and some lizards watch in the neigh- borhood of the place where their eggs are con- cealed, and the python has been seen in mena- geries coiling herself around her eggs in "a conical form, closing the top with her head. Some of the serpents are viviparous, the young being sorfar developed before the exclusion of the eggs as to be born alive ; in the viviparous snakes the young are said to take refuge within the mouth of the mother. The eggs have gen- erally a more or less calcareous shell, globular or rounded equally at each end; in serpents they are often joined together in chaplets ; their number varies from 20 to 100. The em- bryo is completely enveloped by the amnios, and after it has attained a considerable degree of development a second membranous covering appears, for the first time in vertebrates, the allantois, richly supplied with vessels and en- closing embryo and amnios. As reptiles are generally despised and hated by man, and com- paratively little under his influence, their ori- ginal geographical distribution has been but slightly changed by him. Most of the serpents, especially the venomous kinds, belong to warm regions. The secondary geological epoch, com- prising the trias, Jurassic, and chalk, has been called the age of reptiles ; during this period air-breathing animals first appeared in consid- erable numbers, and reptilian forms predom- inated. Reptiles are connected with birds, especially those of the former called sympliy- poda by Cope; dinosaurians, progressing by leaps, with very small anterior limbs, have made many of the bird-like tracks described by Hitchcock in the sandstone of the Connecticut valley. The gigantic and uncouth forms of the secondary age had disappeared in the tertiary, and the reptiles of the latter were more like the present ones, except in geographical distri- bution, and were in about the same proportion to the rest of creation as now. The study of fossil reptiles shows the limited duration of species; before the diluvial -epoch there is not a single reptile that can be referred to living species and hardly to an existing genus; the reptiles of each age, triassic, Jurassic, and cre- taceous, have a special facies, unlike any which preceded or followed them ; the difference be- tween the fossil and living forms is always greater as we go back in time. This study also proves that the temperature of the earth has varied, as the great reptiles above named
