206 ANNELIDA. 



that, notwithstanding the similarity observable in the distribution 

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

 logical point of view they are subservient to very different func- 

 tions ; the former representing the systemic, the latter the pul- 

 monary 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 furnishing 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 integument, but at 

 the same time is brought in contact with the air as it passes through 

 the cutaneous net-work, and is thus, more or less, replenished 

 with oxygen before it is again returned to the general circulation. 

 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. 



(248.) We see, therefore, that the extensive diffusion of vascular 

 canals immediately beneath the surface of the skin must undoubtedly 

 contribute materially to effect those changes in the blood which 

 are analogous to those produced by respiration in the higher ani- 

 mals ; but it would seem that this is not the only provision made 

 for the aeration of the circulating fluids. It is long since Willis* 

 described the existence of a series of pores upon the back of the 

 earthworm, which he regarded as stigmata, and had remarked 

 that air blown into these openings is dispersed between the mus- 

 cular integument and the intestine, so that it passes readily from 

 one segment to another. Duges repeated these experiments with 

 the same result, and found that the pores alluded to, instead of 

 terminating in muciparous follicles, as they were supposed to do 

 by many, penetrate into the interior of the body, so that air in- 

 jected into one of them passes freely along the membranous com- 

 partments which surround the intestine, and escapes through other 

 neighbouring orifices. In like manner water is found to be 

 taken into the body through the same apertures, from which it is 

 often given out in great abundance when the animal is too rapidly 

 dried by exposure to the sun, or irritated by external stimuli: 



* De Ahima Brutorum, 4to. 1672. 



