234 LECTURE XI. 



it favours circulation by obviating the pressure upon the blood- 

 vessels, which would follow the contact of the intestine with the 

 integument, and is, perhaps, the source, or one of the sources, of the 

 blood itself. Besides the organic and definitely shaped corpuscles, it 

 contains albumen, fibrine, and also crystalline products identical 

 with those obtained by evaporation of pure sea-water, and, therefore, 

 merits, as in the Polypes and Echinoderms, the name of the chyl- 

 aqueous fluid. Dr. Williams believes that " sea-water is vitalised 

 with wonderful facility by the solid organic elements contained in 

 the chylo-peritoneal fluid." * 



In the majority of the Anellids the blood is red; in some of a 

 brilliant red, as in Are7iicola, Nereis, Glycera, Nephtys. In Aphro- 

 dita and Polynoe, the blood is of a pale yellow colour ; in a species 

 of Sahella it is olive green ; so that, as Milne Edwards well observes, 

 the colour of the blood is far from being a character of such phy- 

 siological importance as to justify the location of the Anellides at the 

 head of the articulate sub-kingdom. Dr. Williams denies that the 

 blood of the Anellides contains any coloured or other corpuscles. 

 It circulates in a closed system of arteries and veins, the modifi- 

 cations of which are considerable when examined in the different 

 genera of the class. 



A large vessel which runs beneath the dorsal integument is the 

 seat of undulating contractions, by which the blood is propelled from 

 behind forwards ; it fulfils the functions of the heart, and is the 

 homologue of the dorsal vasiform heart in insects. A corresponding 

 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. 

 These are united at each segment by transverse vessels, which 

 convey the fluid from the ventral vein to the dorsal artery: the 

 latter or annular 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 anellids. 



In the leech the vascular system consists principally of four great 

 trunkSj 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 transverse section, as connected together in two of the middle 

 segments of the leech in the diagram {Jig. 105.), from Brandt's Mo- 

 nograph. f The lateral 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 



* CXC. p. 173. t CLXXXV. 



