as they approach the terminal sucking-disk. The fifth is situated on or near the male genital organ; the sixth is partly obscured by the uterus and ovaries of the female apparatus. At the tail there is a larger one, which gives off numerous branches to the prehensile disk.
Dr. Brandt has described in the medicinal leech a single filament which appears to be given off from the œsophageal ganglion, and runs along the dorsal surface of the body, and is distributed to the alimentary canal. This Mr. Owen regards as the first trace of a distinct system of nerves, usually called the stomato-gastric in Entomology, and to which our great sympathetic and nervus vagus seem answerable.[1] At present I have been unable to find such a nerve in the horse-leech.
Respiratory System.—The respiratory system of the leech tribe is, even at the present day, involved in considerable obscurity, notwithstanding the various opinions whi'ch have been offered on this subject: the chief points of dispute being, whether the function of respiration is carried on by the skin, or by an apparatus consisting of loop-shaped glands and receptacles, which pour out on the abdominal surface of the body an abundance of a mucus-like secretion. On each side of the animal (fig. 9), besides the genital organs and the lateral vessel, may be observed a row of small loop-shaped bodies, and a similar number of membranous sacs in the form of bladders connected with them, and containing in their interior a semi-fluid substance, which becomes more liquid after the death of the animal, and especially when slight decomposition has taken place. These bodies communicate each with a small short duct, which opens externally in the skin by a minute aperture, which may be seen with a pocket lens at every fifth ring, (fig. 5). These openings are rather difficult to discover at first, but when one has been seen the others can readily be made out, by counting the rings and looking at every fifth; or when the leech has been wiped dry by means of a cloth, and then examined, small drops of fluid will be found to issue from each of these pores. These bodies are much more evident in the medicinal leech than in the species under consideration, and they are at all times rather difficult to see, especially if the fluid they contain should have been suffered to escape. They are highly vascular, and on this account they have been supposed by some authors to be the respiratory organs, and the fluid they secrete has been said to be analogous to the pulmonary exhalation in the higher classes. Other observers assign to these
- ↑ Lectures on Comparative Anatomy, p. 142.
