12 GILJiERT C. UOURNE. 



with its narrow limits in Pleurotomaria, Haliotis^ or Trochus, 

 is correlated witli the tendency towards a secondary bilateral 

 symmetry, the development of two colutnellar muscles, and 

 the position of the cteiiidia wide apart from one another on 

 the right and left sides of the body. The necessary result is 

 an increased breadth of the body, and the blood returning to 

 the heart by the efferent branchial vessels has to traverse a 

 considerable distance before reaching the ventricle. In other 

 words, the auricles are considerably elongated, and the peri- 

 cardium has to be extended to receive them. Very similar 

 relations are seen in Fissurella. 



The heart and circulatory system. — The ventricle 

 is placed rather far forward on the rectum ; no further for- 

 ward than in Fissnrella, but much further forward than in 

 either Pleurotomaria or Haliotis. The walls of the ventricle 

 are so thin and feebly muscular that they are difficult to 

 recognise, even with the highest powers of the microscope. 

 The auricles also have very thin walls but are more easily 

 recognisable. The left auricle is relatively very large (fig. 10), 

 and its anterior border gives off a number of short and wide 

 sinuses, which penetrate the folds of the wall of the left 

 kidney and vascularise this organ. The right auricle is of 

 smaller size. The course of the blood-vessels, as far as I was 

 able to determine it, is of the usual diotocardiate type, and is 

 diagrammatically represented in fig. 4, which is fully lettered 

 and needs no further description. I was unable to trace the 

 course of the aorta, but the blood, after passing to the foot 

 and the various viscera, is evidently collected in a large sinus 

 lying below the pedal ganglia, and is returned to the afferent 

 branchial vessels by sinuses running over the dorsal side of 

 the great mass of muscle-fibres which diverge on each side of 

 the foot to form the columellar muscles. 



The kidneys.— The left kidney (figs. 8, 9, and 13) is of 

 comparatively large size, but its structure and histological 

 characters leave no doubt that it corresponds physiologically 

 to the papillary sac of the Pleurotomariidae, Haliotidas, and 

 Turbouid^e, for it is unquestionably phagocytic and not depu- 



