Fig. 245. - Blood-corpuscles of Vertebrata. a Red blood-discs of man; b Blood-discs of Goose; c Crocodile; d Frog; e Skate.
The purification of the blood is carried out in all Vertebrates by means of distinct respiratory organs, assisted to a greater or less extent by the skin. In the Fishes, and in the Amphibians to some extent, the process of respiration is carried on by means of branchiae or gills - that is, by organs adapted for breathing air dissolved in water. These are therefore often spoken of as "Branchiate" Vertebrates; but the Amphibians always develop true lungs in the later stages of their existence. In the Reptiles, Birds, and Mammals, branchiae are never developed, and the respiration is always carried on by means of true lungs - that is, by organs adapted for breathing air directly. These are therefore often spoken of as the "Abranchiate" Vertebrates.
The waste substances of the body - of which the most important are water, carbonic acid, and urea - are got rid of by the skin, lungs, and kidneys. Under ordinary circumstances, the lungs are mainly occupied with the excretion of carbonic acid and watery vapour. The skin chiefly gets rid of superfluous moisture, but can also in many animals excrete carbonic acid as well. The kidneys are present in almost all Vertebrate animals, and their function is mainly to excrete water, and the nitrogenous substance known as urea. In the majority of cases the fluid excreted by the kidneys is conveyed to the exterior by means of two tubes known as the ureters, which empty themselves into a common receptacle, the urinary bladder. In some cases, however, the ureters open into the termination of the alimentary canal (rectum).
Fig. 246. - Diagram of the circulation of a Mammal. The venous system is marked black ; the arterial system is left white. a Right auricle ; v Right ventricle; p Pulmonary artery, carrying venous blood to the lungs; pv Pulmonary veins, carrying arterial blood from the lungs; a' Left auricle ; v' Left ventricle ; b Aorta, carrying arterial blood to the body; c Vena cava, carrying venous blood to the heart.
The nervous system of Vertebrate animals usually exhibits a well-marked division into two parts - the cerebro-spinal system, and the sympathetic system. The cerebro-spinal system of nerves constitutes the great mass of the nervous system of Vertebrates, and usually exhibits a well-marked separation into spinal cord (myelon) and brain (encephalon). The proportion borne by the brain to the spinal cord differs much in different cases; and in the Lancelet a brain can hardly be said to be present at all. As already said, the brain and spinal cord are always completely shut off from the visceral cavity, and they are placed upon the dorsal surface of the body. The nerves given off from the cerebro-spinal axis are symmetrically disposed on the two sides of the body, and they are mainly concerned with the functions of "animal" life - that is to say, with sensation and locomotion. The sympathetic system of nerves is unsymmetrically disposed to a greater or less extent, and presides mainly over the functions of "organic" or "vegetative" life, being chiefly concerned with regulating the functions of digestion and respiration, and the circulation of the blood. In its most fully developed form it consists of a double gangliated cord placed in the visceral cavity on the under surface of the spine, and of a series of nervous ganglia, united by nervous cords, and scattered chiefly over the great viscera of the thorax and abdomen.
The organs of the senses are well developed in the Vertebrata, and those appropriated to the senses of sight, hearing, smell, and taste are protected within bony cavities of the head. The perfection of the senses differs much in different cases, but they are probably never wholly wanting in any Vertebrate animal. There are cases in which vision must be of the most rudimentary character; but even in these cases it is probable that there is a perception of light, even if there is no power of distinguishing objects. The only cases in which it would appear that vision is really altogether absent, are those of animals placed under the wholly abnormal condition of spending their existence in darkness (such as the Proteus anguinus of the caves of Illyria). Smell, hearing, and taste are probably rarely, if ever, altogether absent in Vertebrates; though in many cases their organs are very rudimentary. Touch, or "tactile sensibility," is usually possessed to a greater or less degree by the entire surface of the body; but the sense of touch is generally localised in certain particular parts, such as the appendages of the mouth, the lips, the tongue, or the digits.
In all Vertebrata without exception reproduction is carried on by means of the sexes, and in all (except in some of the Serranidae among the Fishes) the sexes are in different individuals. No vertebrate animal possesses the power of reproducing itself by fission or gemmation; and in no case are composite organisms or colonies produced. Most of the Vertebrates are oviparous - that is to say, the ova are expelled from the body of the parent either before or very shortly after impregnation. In other cases, the eggs are retained within the body of the parent until the young are hatched, but no direct connection is formed between the foetus and the mother, and in these cases the animals are said to be ovo-viviparous. In other cases, again, not only is the egg hatched within the parent, but the embryo is retained within the body of the mother, from whom it receives nourishment by direct vascular connection, until its development has been carried out to a greater or less extent; and these animals are said to be viviparous.