Page:The Zoologist, 1st series, vol 1 (1843).djvu/355

Rh occurring in the medicinal leech. This is made out in the following way. Those parts of the glandular system which will be described as the loop-shaped glands, are considered to be blood-vessels coming off from the lateral trunks; these carry the blood to the walls of the raucous sacs, where it is brought into contact with the aerated water contained in the sacs, and from which it is again conveyed to the lateral trunk by means of a small communicating vein.

Sensation.—Of the five senses which are common to animals of the higher classes, that of sight is the most perfect in the horseleech. The eyes of this creature are ten in number, and are placed on the dorsal surface of the anterior sucker, (fig. 3): six of them are arranged in a curved form near the margin of the disk, three on each side of the median line, with a slight space between them; behind these, and at a little distance, we have two others, and at a still greater distance of a line and a half, are the remaining two, which are much larger than any of the preceding. They are hardly perceptible to the naked eye, but when examined microscopically with a low power, they appear as so many black dots, and with a power of 100 linear the anterior part of the eye is seen not to be black, but transparent; the posterior part is covered with a black pigment, which corresponds to the choroid in the eyes of other animals (fig. 7, 8). The optic nerve enters at the back, and the whole globe of the eye seems to be nothing more than the dilated or bulbous extremity of the optic nerve. Müller, who has examined the eyes of the Annelida with the greatest care, finds some of them to be spherical, but in the leech tribe they appear to be rather of an oval figure. The sense of touch is likewise present to a considerable extent in these animals; but they appear to be quite deficient in that of hearing.

Nervous System.—The nervous system (as in the other members of the class Annelida), may be said to consist of a bilobed ganglion or brain, situate near the head, and a cord composed of two columns, which extends from the brain to the caudal extremity, all lying on the abdominal surface of the body, upon the alimentary canal (fig. 1). The brain is situated about two lines from the mouth, its anterior portion being circular, whilst its posterior is more of a triangular figure; both are firmly imbedded in the thick muscular tissue appended to the mouth