CIECULATION IN THE EAKTHWOEM. 233 



is continually returned to the dorsal canal by the deep and superficial 

 abdomino- dorsal vessels (e, f, g), completing the vascular circle. 



(605.) On reviewing the above arrangement, we immediately perceive 

 that, notwithstanding the similarity observable in the distribution of 

 the ventral and sub-ganglionic systems of vessels, in a physiological 

 point of view they are subservient to- very different functions the 

 former representing the systemic, the latter the pulmonary circulation. 

 The blood derived from the dorsal trunk by the moniliform hearts (d) 

 is supplied by the ventral vessel (which may be compared to an aorta), 

 over the surface of the viscera ; and the remnant of this blood, after fur- 

 nishing materials for nutrition, is returned to the dorsal canal by the 

 deep vessels (e, g] ; but that portion of the circulating fluid which passes 

 from the termination of the dorsal tube into the sub-ganglionic trunk 

 not only serves for the nourishment of the skin and muscular integu- 

 ment, but at the same time is brought in contact with the air as it 

 passes through the cutaneous network, and is thus more or less re- 

 plenished with oxygen before it is again returned to the general circu- 

 lation. The sub-ganglionic canal is therefore a kind of pulmonary 

 artery, and the dorsal drives to the moniliform vessels a mixed fluid, 

 composed partly of venous blood derived from the viscera, and partly of 

 arterial derived from the superficial or sub -cutaneous system. 



(606.) Consequently in the Lumbrici the primary longitudinal trunks 

 are similar in number and disposition to those of the Leech, and 

 the direction of the blood- current is almost the same. The intestine 

 in the Earthworm (in this respect distinguished from the Leech) is only 

 tied to the integuments at the interannular points, the intervals or 

 segmental spaces being left as chambers (fig. Ill, v v) containing a 

 small quantity of viscid corpusculated fluid, which is the peritoneal fluid 

 of this worm. The interposition of a fluid stratum in this part involves 

 other anatomical modifications, which still further separate the organi- 

 zation of the Earthworm from that of the Leech ; the spongy vessels 

 described above as occupying this part in the latter are absent in the 

 former. 



(607.) Superadded to the primary median blood- channels (fig. 113, 

 a, 6, c, d}, a minor lateral system, founded upon the latero-abdominal 

 trunk, may be demonstrated in the Earthworm as in the Leech : in all 

 essential particulars, in the two cases, the main trunk of the system and 

 its branches are the same. 



(608.) Few points connected with the history of the Earthworms 

 have given rise to so much speculation as the manner of their repro- 

 duction. The generative organs have long been known to be lodged in 

 the anterior part of the body, their position being indicated externally 

 by a considerable enlargement or swelling that extends from the seventh 

 to about the fourteenth segment, counting from that in which the mouth 

 is situated. On opening this portion of the animal, a variable number of 



