182 REPORT — 1851. 



These observations of Willis and Duges are totally irreconcileable with the 

 facts adduced in this Report: — first, the alleged orifices (stigmata), commu- 

 nicating directly with the cavity of the body, cannot now be proved to exist; 

 and secondly, it is susceptible of demonstration that the fluid contained in 

 the peritoneal space is not watery in this worm, even though it may have 

 been immersed for some hours in water previously to the examination. As 

 already described, the contents of the cavity are composed of a viscid, cor- 

 pusculated fluid, insusceptible, from its consistency, of such rapid removal as 

 is implied in the above observations. The orifices which communicate with 

 the interior in this worm, as in nearly all others, open directly into the mem- 

 branous utriculi of the generative system. No other perforations can be proved 

 to exist. 



In Nats filiformis, so abundant in the freshwater pools of this country, 

 the anatomist 'is presented with a favourable opportunity for resolving the 

 problem of the circulation (fig. 8). A living specimen, placed between two 

 slips of glass, from the perfect transparency of the integuments, will exhibit 

 to the eye, in a perfect manner, all the circulating movements both of the 

 vessels and the blood. In Nais, the large dorsal vessel (fig. 8, a) is first seen 

 travelling wavingly along the dorsum of the intestine as far as the heart, 

 which corresponds in situation with the intestinal end of the oesophagus. 

 This vessel is enveloped by the glandular peritoneal layer of the intestine, 

 while the coats of the ventral vessel are clear and transparent ; the dorsal 

 vessel is endowed with parietes of greater strength and density than the 

 ventral. Each of these vessels (as at fig. 8, a', b') dilates into a fusiform heart, 

 which is situated on either side of the oesophagus. These hearts, which are 

 joined together by transverse vessels, pulsate alternately, and with exact 

 regularity. In the dorsal vessel the blood moves forwards from the tail, as 

 far as the dorsal heart ; thence it descends into the ventral heart, by which it 

 is now propelled, chiefly in a backward direction, partly through the main 

 ventral trunk, and partly through the inferior intestinal. The other portion 

 of the blood, conveyed by the great dorsal vessel into the ventral heart (6'), 

 passes forwards as far as the head, where its moving power is again rein- 

 forced by a cardiac dilatation, which now impels the current from before 

 backwards through a superior oesophageal trunk into the dorsal heart (a'), by 

 which organ, the blood, received from the region of the oesophagus, and coming 

 from the head, as well as that received from the great dorsal, and coming from, 

 the tail, is urged downwards into the ventral heart, and thence, chiefly in the 

 direction of the tail, through the ventral and intestinal trunks {e,f); this 

 latter, therefore, is the true systemic heart. At the oesophageal end of the 

 body the two primary trunks, dorsal and ventral, are connected together by 

 means of a remarkable class of vessels (g, f/, like g, g', g), which in this region 

 proceed at successive points from the dorsal oesophageal, and which may be 

 traced in long coils, without division of the vessel, floating in the fluid of the 

 peritoneal cavity. Posteriorly to the heart-centre these vessels (fig. 8, g, g, g) 

 emanate from the dorsal intestinal (b), and correspond precisely with those 

 branches from the same vessel, which in Arenicola Piscatorum proceed to 

 supply the branchial arbuscles. In Nais, therefore, partly from this ana- 

 logy, but chiefly from their anatomical relations, bathed by, and floating in, 

 the chyl-aqueous contents of the peritoneal cavity, the physiologist can ex- 

 perience no difficulty in dedicating these coiled vessels to uses very definite. 

 First, it cannot be doubted that they absorb from this fluid the elements by 

 which the blood-proper is formed and replenished ; and secondly, it is in the 

 strongest degree probable, that the true blood is in great part &ei'a.ted through 

 the agency of these vessels upon the gaseous elements contained in the peri- 



