A \NI<: 1.1 1) \. 1 1)7 



The two systemic trunks ( /', g) run along the mcsian line of 

 l.lir body ; one upon the <lors.il, ;in<l llir oi.lirr upon the ventral 

 as|>cet. The dorsal vessel (y) seems to be ;i.rl.eri;i.l in its eha- 

 raetrr, and no doubt corresponds in function with the heart of 

 more |>nfeet forms of the articulata ; receiving the blood from all 

 parts of the system, as'well from the respiratory vessels as from 

 the venous capillaries, and by successive undulatory contractions, 

 uhieh may be observed to proceed from the tail towards the an- 

 terior extremity, propelling it through all the arterial branches 

 derived from it. The ventral vessel (g), on the contrary, seems 

 to be venous, collecting the blood after its passage through I he 

 systemic capillaries, and returning it partly into the dorsal artery 

 from wliieli it set out, and partly to the lateral vessels for the 

 purpose of undergoing respiration. 



The two lateral vessels (a, c) are appropriated to the supply 

 of the respiratory system, and in them the blood moves in a circle 

 quite independent of that formed by the dorsal artery and ventral 

 vein, although they all communicate freely by means of cross 

 branches, those passing from the lateral vessels to the dorsal 

 being called by M. Duges* dor so- lateral , while those which 

 join the lateral trunks to the ventral canal arc the latero-abdo- 

 minal branches of that observer. The movement of, the blood 

 in the lateral or respiratory system of vessels is quite distinct 

 from that which is accomplished in the dorso- ventral or systemic 

 trunks : sometimes it passes down the vessel marked a, from the 

 head towards the tail, and in an opposite direction on the other 

 side of the body ; but in a short time the movement of the cur- 

 rents will be seen to become completely reversed, so that an un- 

 dulatory motion, rather than a complete circulation, is kept up. 

 By this action of the lateral canals the blood is made perpetually 

 to pass and repass the respiratory sacculi ; and, opposite to each 

 of these, branches are given off which form so many independent 

 vascular circles, representing very closely the minor or pulmonary 

 circulation of higher animals. 



(239.) On examining attentively one of the respiratory pouches 

 (Jig. 81, y ), its membranous walls arc seen to be covered with very 

 line vascular ramifications, derived from two sources: the latcro-ab- 

 dominal vessel (d) gives off a branch (c), which is distributed upon 

 the respiratory sacculus ; and there is another very flcxuous vas- 



* Annalcs dcs Sciences Nat. vo). xv. 



