Annelides. 329 



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 lar- 

 ger 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 oesophageal ganglion, and runs 

 along the dorsal surface of the body, and is distributed to the alimen- 

 tary canal. This Mr. Owen regards as the first trace of a distinct sys- 

 tem of nerves, usually called the stomato-gastric in Entomology, and 

 to which our great sympathetic and nervus vagus seem answerable.* 

 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, notwith- 

 standing the various opinions which have been offered on this sub- 

 ject : the chief points of dispute being, whether the function of re- 

 spiration 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 espe- 

 cially 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 dis- 

 cover 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 diffi- 

 cult to see, especially if the fluid they contain should have been suf- 

 fered 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 pulmo- 

 nary exhalation in the higher classes. Other observers assign to these 



Lectures on Conipamlive Anatomy, p. 142. 



