CIR'CULATO'RY SYSTEM, EVOLUTION or. The organs by whieh fluids and sometiines gases and solids. suspended in the fluids. are trans ported from one part of the body to another. A circulatory system first becomes necessary when the organism gets to be of great size and a specialization of structure has taken place, so that the nutrient regions where fluid and gas eous fond are supplied are far removed front the muscles, glands. and other organs which re quire that food, and \Owno the excretory organs are distant from the great centres of metabolism. The circulatory system fulfills the function of transportation between the muscles and glands on the one hand, and the alimentary tract, res piratory organs. and kidneys on the other.
Loire•t Forms.—In the Protozoa the body is so small that no special apparatus necessary. The cell-structure with its water spaces provides for carrying food in solution to the remotest plasm films. In the sponges the body often attains great size, lint by the system of water-canals penetrating all parts of the mass, food and oxygen are directly brought, as it were. to the door of every cell. Even in the enidaria the body-wall is typically only two cells thick, so that between the external fluids and those of the cavity every cell is brought into direct contact with oxygenated water. In the where a true body-cavity between the alimentary tract and the body-wall makes its appearance. the fluids of this cavity serve to carry nutritive fluids and excreted products. The fluids of the bodv-cavity are often set in nod ion by the cilia of the lining cells. to the Alollusca the !Jody is so large and tile muscular system so well developed that a Inure effective transporting system is necessary. This is gained by the separation of special tracts of the body-cavity. In the dorsal part of the animal is a space that has gained thick muscular walls and constitutes a propelling organ or heart. The alimentary tract usually perforates the heart so that nutri tive fluid: by passing through the wall of the gut may reach the circulating fluid. From the heart a tubular space carries the circulating fluid to the spaces of the mantle and of the muscles and glands. These spaces are parts of the body
cavity and, as in the ease of the foot, some of them even connect with the outside world. The fluids in these spaces and in the gills make their way. probably by the aid of cilia, to the heart. Oxygen is obtained bade in the thin walled mantle and in the gills. The Alollusea show thus a great advance in the specialization of a definite system of blood-vessels. The echino derms have acquired much the same or a less perfect system of blood-spaces.
Arthropods.—Among segmented animals we find that many of the Pulp.Ineta have a well-de lined system of blood-vessels. which they have perhaps derived from the nemerteans. The system shows an advance over that of nemerteans, how ever: for in the latter group there is no regular circulation, whereas in the Polyclueta such a cir culation is established. They have a main dorsal vessel and a median ventral vessel, and a circular transverse vessel running in the body-wall be tween the two. 'Vessels going to the alimentary tract get nutritive fluids and those going to the hody-wall. pa rapodia. and gills bring hack oxygen. The blood-vessels contain corpuscles and often Inemoglobin. The dorsal-vessel 'heart' contracts Not all Polychwta have a com plete (closed) blood system. and in some eases the body fluid is the only circulating medium. The earthworms also have a well-developed sys tem of vessels.
In the Crustacea there is usually a dorsal-ves sel 'heart.' but well-defined arteries and veins are often lacking. the heart pumping the general fluids of the body-eavity. however, in the higher Crust:ice:1, as, e.g. the lobster, definite vessels to the brain, muscles, gills, alimentary trait appear. from which organs the blood is re turned to the heart by blood-spaces. In insects the respiratory functions of the circulatory sys tem largely replaced by respiratory tubes which ramify even to the muscles. There is al ways a heart, however. and in Arachnoidea a number of well-defined blood-vessels.