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.
[Illustration: Fig. 131. Nervous System of Leech.]
The circulation of the blood and of the colourless liquid, as well as the nerve system, prevail generally in the Annelids, modified by the structure of the individual.
The leech, though greedy of blood, lives in fresh-water ponds, wet grass, and damp places, where it never can meet with warm-blooded animals. It probably lives on minute aquatic insects.
The common Earth-worm, which is a principal member of the second order of Annelids, has a more important part assigned to it in the economy of nature than its humble appearance leads us to suspect. It has a long, soft, cylindrical body tapering to a point at both ends, divided into numerous rings. The mouth is furnished with a short proboscis, or snout, without teeth. A long salivary glandular mass surrounds a short wide gullet, which leads to a digestive organ similar to a gizzard, whence a canal is continued to the vent. The circulation of the two fluids, and the nervous system modified at head and tail, are like those of the leech. Four rows of minute bristles extend longitudinally along the ventral surface of the worm, two on each side. With a low magnifying-power they appear to be minute points regularly pushed out and drawn in; but when more highly magnified each point is seen to consist of two transparent glassy rods having their points bent backwards: on these feet the worm crawls very rapidly.
While making its cylindrical burrow a slimy mucus exudes from the body of the worm, which cements the particles of earth together and renders the walls of the burrow perfectly smooth and slippery. When the worm pierces the earth it stretches its snout into a fine point that it may penetrate more easily, and when it is fixed, it draws its ringed body towards its head by a muscular effort; and to prevent it from slipping back again, it fixes the hooks of its posterior feet firmly into the ground. Having thus secured a point of support it penetrates deeper into the earth, draws up its body, fixes the hooks of the posterior feet into the smooth surface of the burrow, and continues the same process till the burrow is deep enough. Thus the feet are employed as points of resistance for the exertion of muscular force. This worm swallows earth mixed with decaying animal and vegetable matter, assimilates the nutritive part, and casts out the refuse in the form of fine mould, which may be seen in little heaps at the edges of their burrows. In fact, nearly all the fine vegetable mould so precious to gardeners and farmers has passed through the intestines of the common earth-worm.