ANNELIDA TERRICOLA. 



the latter the pulmonary circulation. The blood derived from 

 the dorsal trunk by the moniliform hearts 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 ; 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 cuta- 

 neous network, 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 vis- 

 cera, and partly of arterial, derived from the superficial or sub- 

 cutaneous system. 



Thus, therefore, the extensive diffusion of vascular canals 

 immediately beneath the surface of the skin must undoubt- 

 edly contribute materially to effect those changes in the blood 

 which are analogous to those produced by respiration in the 

 higher animals ; but it would seem that this is not the only 

 provision made for the aeration of the circulating fluids. It is 

 long since Willis (1672) described the existence of a series of 

 pores upon the back of the earth-worm, which he regarded as 

 stigmata, and had remarked that air blown into these open- 

 ings is dispersed between the muscular integument and the 

 intestine, so that it passes readily from one segment to an- 

 other. Duge's repeated these experiments with the same 

 result, and found that the pores alluded to, instead of termi- 

 nating in muciparous follicles, as they were supposed to do 

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

 injected into one of them passes freely along the membra- 

 nous compartments which surround the intestine, and escapes 

 through other neighboring 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. Aerated water, thus taken into the system, 

 and brought immediately in contact with the deep-seated 

 vascular net- work dispersed over the intestinal parietes, must 

 therefore necessarily contribute to the respiratory function. 



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