774 PKOF. G. C. BOURNE ON THE 



the habits of the various species of Heliciniflse, and do not even 

 know for certain whether each or any species is restricted to a 

 particular kind of food, it would be rash to speculate on this 

 question, but such evidence as I have collected does seem to show 

 that such apparently trivial characters as the coils of the intestine 

 are of some physiological importance, and are therefore subject to 

 the action of natural selection. 



The Codom. 



The coelom is represented by the pericardial cavity, which, 

 although it is of much smaller extent and less complicated than in 

 the Neritidae, is nevertheless a cavity of relatively considerable size, 

 as may be seen by an inspection of figs. 17 to 20 (PI. XXXIII.). 

 As is shown \\\ fig. 1 (PI. XXX.) it comes close to the surface of 

 the left side of the visceral mass, and extends forwards neai^y 

 as far as the posterior end of the left columellar muscle and 

 backwards round the lower side of the viscei'al mass as far as the 

 end of the mantle-cavity. It is bounded externally by the very 

 thin body-wall, posteriorly by the inner wall of the mantle- 

 cavity, internally by the kidney (figs. 17 to 20). At abovit the 

 middle of its length it is a cavity of considerable depth, extending 

 some way into the visceral mass below the pyloric division of 

 the stomach. The reno-pericardial canal, which will be described 

 in connection with the kidney, opens into its right posterior wall, 

 at some little distance from its hindermost end (fig. 19); other- 

 wise it is a closed sac containing the auricle and ventricle of the 

 heai't, and does not require further description. 



The Hcemoccele, Circulatory and Respiratory Systems. 



The blood-vascular system, as in all Molluscs, consists partly 

 of large lacunar spaces, which collectively are known as hsemocoele, 

 and partly of vessels with definite walls. The hsemocoelic spaces 

 svirround the viscera, and there is a specially large lacunar space 

 below the buccal bulb, in which lie the pleuro-pedal ganglia. 

 In the visceral mass and in the dorsal region of the body the 

 hsemoccele is largely filled up by the peculiar form of connective 

 tissue which I have previously described (2, p. 861) as metabolic 

 tissue. In the Helicinidfe the tissue is of precisely the same 

 nature as in Neritidae, and it is not necessary to describe it again. 

 It evidently consists in large part of reserve tissue, for it is most 

 abundant in immature specimens in which the gonads and 

 gonaducts are but slightly developed, and is much less abundant 

 in sexually mature specimens. This metabolic tissue is specially 

 concentrated round the larger blood-vessels. 



It is not possible to trace the course of the blood-vessels by 

 dissection of spirit -pi^eserved specimens and only the larger vessels 

 can be traced in sections. The following account of the 

 circulation embodies as much information as I have been able to 



