MORPHOLOGY OF THE HELICINID^. 765 



that the extremity of the visceral mass, lying to the right, is 

 wholly occupied by the liver and gonads. If, now, the reader will 

 lay a tobacco-pipe in front of him on the table, the bowl (repre- 

 senting the oesophageal division of the stomach) to his right, the 

 stem (I'epresenting the pyloric division of the stomach) to his left 

 front, it will occupy much the same position as does the stomach 

 in the figure referred to. If, while the bowl is kept pi-essed 

 against the same spot on the table, the stem is lifted up and 

 rotated through an angle of rather moi-e than 90° till it points 

 over the observer's right shoulder, the whole pipe will have been 

 rotated through an angle which brings it into the j^osition of the 

 stomach of the Helicinidse — the animal being supposed to be 

 placed foot downwards upon the table with its head turned away 

 from the observer. As all the organs of the left side of the 

 body, including the posterior end of the mantle-cavity, the peri- 

 cardium, the heart, the kidney, and the coils of the intestine, 

 have shared in this movement of the pyloric end of the stomach, 

 their positions have been nearly completely reversed, and the 

 left auricle instead of lying in front of the ventricle has come 

 to lie behind. The right auricle has entirely disappeared in the 

 Helicinidse, and the rectum, having undergone some degree of 

 displacement in connection with the above-described movement 

 of rotation, is no longer enveloped by the ventricle. 



The kidney has also undergone a curious and at first sight a 

 puzzling change of position. In the Neritidte, as described in 

 my previous memoir (2, see PI. XLVI. fig. 1 for its position in 

 Septaria, PI. LIV. fig. 29 for its position in Paranerita gagates), 

 its glandular part lies to the left hand of and partly below the 

 rectum : posteriorly the glandular part opens into the sj)acious 

 non-glandular bladder or ureter, and the latter runs forward, 

 below the glandular part, to open by the uropore into the mantle- 

 cavity on the right of and close to the base of the ctenidium. 

 In this family the greater part of the kidney lies in the roof 

 of the mantle ; it is only its posterior extremity that passes below 

 the rectum and invades the visceral mass, where it lies just above 

 the pyloric end of the stomach. The effect of the rotation of the 

 last-named organ in the Helicinidse is that the kidney has been 

 carried i-ound till it comes to lie wholly in the visceral mass, on the 

 lower side of the latter, between the loop of the rectum which 

 passes round this region and the pyloric division of the stomach, 

 as may be seen in the series of sections (PI. XXXIII. figs. 1 7 to 20) 

 and in the diagram (PI. XXXIY. fig. 24), which is a reconstruction 

 from this series of sections. The kidney, in short, has been 

 turned completely round, so that its originally posterior end looks 

 to the left front and the uropore opens into the right hinder 

 corner of the mantle-cavity, the reno-pericardial canal, main- 

 taining its relation to the uropore, into the right posterior corner 

 of the pericardium. It is further to be observed that the visceral 

 mass, though apparently more coiled, is really less coiled in the 

 Helicinidte than in the Xeritidse. In all systematic works stress 

 Proc. ZooL. Soc— 1911, No. LIII. 53 



