CIKCULATION IN NAIS. 



241 



fore 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 g g), which in this region pro- 

 ceed at successive points from the dorsal oasophageal, 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 

 emanate from the dorsal intestinal, 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 

 analogy, but chiefly from their anatomical relations, bathed by and 

 floating in the chylaqueous contents of the peritoneal cavity, the phy- 

 siologist can experience 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 reple- 

 nished ; and secondly, it is in the strongest -degree probable that the 

 true blood is in great part aerated through the agency of these vessels 

 upon the gaseous elements contained in the peritoneal fluid. They 

 constitute the special branchial system (internal 

 branchiae), while they discharge incidentally an 

 absorbent function. In the movement of the 

 blood, then, in Nais as in Lumbricus, there are 

 discernible only two leading directions one 

 forward in the primary and intestinal dorsal 

 vessels, the other backward in the primary and 

 intestinal ventral. It is not possible to trace 

 the blood into the capillary parietal system of 

 the intestine, in consequence of the transpa- 

 rency of the stream when thus minutely sub- 

 divided. In Nais there is also an integumentary 

 system which intervenes between the two pri- 

 mary (dorsal and abdominal) trunks (#,/), 

 ramifying in the substance of the integuments, 

 upon which, in part, a respiratory function may 

 devolve. 



(632.) The generative system of the Nais, 

 as delineated by Duges, presents a very dif- 

 ferent arrangement to that which exists in the 

 Earthworm. The swollen part of the body, in 

 which the sexual organs are placed, occupies a 

 space of five or six rings, beginning at the 

 eleventh. On each side of the eleventh seg- 

 ment is a minute transverse slit (fig. 117, 6) 

 communicating with a slightly-flexuous canal 

 which terminates in a transparent pyriform 

 pouch or vesicle. The latter contains a clear fluid, wherein minute 



Plan of circulation in Nais. 

 (After Dr. Williams.) 



