The first order consists of Leeches of different kinds: their body is long, slightly segmented, with a suctorial disc at each end. Their skin is smooth, whitish, and translucent; beneath it are cells filled with brown or greenish matter, and three layers of muscular fibres follow; the first are transverse, the second cross one another diagonally so as to form a network, and the third are longitudinal. The mouth, which occupies the centre of the principal sucking disk, varies in form with the genera. In the common leech it has an enlarged lip, and opens into a short gullet leading into a capacious and singularly complicated stomach, divided by deep constrictions into eleven compartments, the last of which is connected with an intestinal canal, which ends in a vent in the middle of the terminal sucker. Within the mouth there are three crescent-shaped jaws, presenting their convex edges towards the cavity of the mouth, beset with from seventy to eighty teeth, formed of a highly refractive crystalline substance resembling glass. The leech makes a vacuum with its sucker, which forces the part to which it is applied into contact with the three-toothed jaws, which are moved sidewise by strong muscles, and saw through the skin and small bloodvessels below it. The leech, like the other Annelids, has two distinct systems of circulating liquids, one red, the other colourless. The red liquid or blood is kept in circulation by the pulsations of a heart, or rather a contractile vessel behind the head. It is carried away from the heart by a pulsating canal passing along the back of the leech, and is brought back to the heart by a similar canal extending along its ventral side. During this course, portions of the liquid are sent off through veins to different parts of the body. The respiratory organs of the leech are pores arranged at regular distances on each side of the body which open into little sacs having capillary bloodvessels distributed under the skin through which the blood is aërated. The colourless liquid which contains many organic molecules, occupies the space between the alimentary canal and the inner wall of the body, from whence it passes into canals which ramify extensively, but are not furnished with returning passages. This liquid forms a support to the muscles of the skin, and is kept in circulation by the motions of the leech. Fig. 131 shows the highly developed nervous system of the leech. From the double lobe of the brain ten optic nerves go to the bases of ten black eye-specks, which mark at equal distances the upper margin of the expanded lip. A nerve-centre below the gullet supplies the lip and jaws with strong nerves. A double longitudinal cord, united at equal distances by twenty-one double nerve-centres, extends from a ring round the gullet throughout the whole length of the body, supplies the different organs with nerves, and ends near the vent in a nerve-centre, from whence nerves radiate through the terminal sucker.