136 



LECTURE XI. 



undulating contractions, by which the blood is propelled from behind 

 forwards ; it fulfils the functions of the heart, and is very obviously 

 the analogue of the dorsal vasiform heart in insects. A corre- 

 sponding venous trunk conveys the blood in an opposite direction 

 from the head to the tail, along the under or ventral surface of the 

 abdominal cavity. The circulation is often aided by the contractile 

 walls of partial dilatations of certain of the vessels, and by the actions 

 of the gills themselves in the higher anellides. 



In the leech the vascular system consists principally of four great 

 trunks, none of which present any local dilatations meriting the name 

 of heart ; one of these trunks is situated on each side, a third above, 

 and a fourth below, the alimentary canal. They are shown in trans- 

 verse section, as connected together in two of the middle segments of 

 the leech in this diagram, from Brandt's Monograph {Jig. 72.) The la- 

 teral trunks (c, c) are the 

 largest ; they are widest 

 in the posterior third of 

 the body ; their anterior 

 end terminates in branches 

 to the head ; the posterior 

 end unites with that of 

 its fellow more conspic- 

 uously than at the an- 

 terior part, and supplies 

 the terminal sucker. 

 Branches are given off 

 at each ring, which almost immediately divide into a dorsal {d) and 

 ventral (e) ramulus ; the six posterior dorsal branches unite with 

 those of the opposite side, and the six arches thus formed are joined 

 together by two nearly parallel longitudinal vessels near the middle 

 line of the back. 



The dorsal vessel {fig. 72. a), in which the blood moves from be- 

 hind 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 for- 

 wards, 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 {fig. 

 72. h) 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 



Leech. 



* Johnson On the Medicinal Leech, Svo. 1816. p. 114. 



