CIRCULATORY SYSTEM OF THE LEECH. 221 



this anatomical fact the highest interest will be shown to belong when 

 explaining the mechanism of respiration in this Annelid. Here the 

 chylous fluid, which in nearly all other Annelids occupies the general 

 cavity of the body, like a cylindrical fluid stratum, separating the intes- 

 tine from the integument, is transferred into the interior of the lateral 

 diverticula of the stomach. The peritoneal chamber, being no longer 

 required, is obliterated by the adhesion of the intestine to the integu- 

 ment : the union of these parts is effected through the medium of a 

 dense spongy layer of capillary blood-vessels, the contents of which are 

 exposed internally to the influence of the fluid contained in the digestive 

 caeca, and externally to that of the surrounding element : hence the 

 mechanism of the respiratory process, and the power enjoyed by this and 

 other abranchiate Annelids of dispensing with all external breathing 

 appendages. 



(579.) While, however, the peripheral segment of the vascular system 

 in the Leech exhibits proofs of great complexity, the main currents of 

 the blood obey two leading directions. If the body of the worm be lon- 

 gitudinally bisected by an imaginary horizontal plane into a dorsal and 

 ventral semicylinder, then the blood in the primary trunks of the 

 dorsal half will move from the tail towards the head, and in the ventral 

 half from the head towards the tail : this movement prevails equally in 

 the great longitudinal trunks of the integuments and alimentary canal. 

 The transverse or circular movement of the blood is performed by 

 means of branches which run between the main longitudinal vessels : 

 this latter system is divisible into as many portions as there are rings in 

 the body of the worm : each segment of the body under this arrange- 

 ment has its own independent circulation, transverse and longitudinal. 

 Thus the currents describe two excentric ellipses, cutting each other at 

 right angles. Of course, the segmental divisions of the general system 

 communicate with each other at every part, while the longitudinal 

 trunks are common to all the segments. From this description it is 

 manifestly impossible that a distinction of venous and arterial blood can 

 exist in the circulating fluid of this Annelid ; in every part of the cir- 

 cumference of each ring the blood is being arterialized as it is being 

 rendered venous ; the two opposite processes proceed simultaneously in 

 the same capillary system. The blood must be, therefore, as arterial 

 and as venous at one and the same time in the dorsal as in the ventral 

 trunks ; notwithstanding, the dorsal main is recipient, the ventral 

 distributive of the blood : all the secondary currents converge upon the 

 former, and emanate from the latter ; the blood in both is nevertheless 

 identical in physiological properties. 



(580.) In addition to the main dorsal and ventral trunks, there exist 

 in the Leech two strong and obvious lateral trunks, one on each side 

 (fig. 108, ee). The branches exhibit in their walls a structure pre- 

 cisely the same as that which distinguishes the vascular system in every 



