ANNUL AT A. 



235 



more conspicuously than at the anterior part, and supplies the ter- 

 minal sucker. Branches 

 are given off at each 

 ring, which almost im- 

 mediately divide into a 

 dorsal {d) and ventral 

 (e) ramulus ; the six pos- 

 terior dorsal branches 

 unite with those of the 

 opposite side, and the 

 six arches thus formed 

 are joined together by 

 near the middle line of 



Leech. 



two nearly parallel longitudinal vessels 

 the back. 



The dorsal vessel {Jig. 105. a), in which the blood moves from behind 

 forwards, is formed by the union of the dorso-intestinal vein and of 

 the dorso-dermal vessel, which run parallel with each other along 

 the posterior third of the body. The trunk is thence continued 

 forwards, sending outwards a pair of transverse branches at each 

 ring, and bifurcating behind the mouth to enclose the oesophagus. 

 From the under part of the oesophageal ring the great ventral vein 

 {Jig. 105. b) begins, which is continued along the nervous ganglionic 

 chord, and swells at each ganglion, forming a sinus around it ; the 

 nervous matter being thus, as it were, bathed in the nutrient fluid.* 

 From each of these swellings a transverse branch is sent off to either 

 side {f,f\ and from the seventh to the fifteenth ganglionic sinus a 

 second pair of transverse vessels of smaller size is given off, just 

 behind the ganglionic sinus. 



The respiratory function would seem to devolve partly upon the 

 tegumentary capillaries, and partly upon those capillaries which 

 spread upon the mucous sacculi. These latter capillaries are not, 

 however, more numerous than those of the other organs of the body. 

 The more important office of the sacculi would seem to be as the 

 recipients of the secretion of peculiar loop-shaped glands, which they 

 receive by a very short and slender duct. There are seventeen pairs 

 of these mucous glands {Jigs. 104, 105. g^ g), the five posterior of which 

 lie on each side the long terminal gastric sacculi, and the rest in the 

 interspaces of the shorter cceca. Each gland pours its secretion into 

 a circular sac {Jigs. 104, 105. h, h), which opens externally {fg. 104. 

 i, i) upon the skin. These dermal pouches have commonly been 

 described as the respiratory organs ; and from their relative position 



* CLXXXIV. p. 114. 



