] 908.] ASPIDOBRANCH GASTROPOD MOLLUSCS. 833^ 



left posterior corner of the foot. Here it receives blood from two 

 venous sinuses : the one bringing back blood from the left posterior 

 region of the body-wall, the other from a considerable system of 

 lacunjB in the posterior part of the foot. The former of these two 

 sinuses runs in close connection with the posterior lobe of the 

 kidney, and it would appear that we have here an arrangement 

 whereby oxygenated blood from the mantle is also enabled to get 

 rid of its waste nitrogenous products before it is returned to the 

 heart. This is the reverse to what occurs in the case of the blood 

 passed through the ctenidiurn, which is first purified of its nitro- 

 genous waste matter during its passage through the sinuses of the- 

 anterior lobes of the kidney and is afterwards carried to the 

 ctenidiurn by the aflferent branchial sinus. The right auricle, like 

 the left, is covered by the glandular tissue of the pericardial gland. 

 As Septaria has undergone a considerable amount of detorsion, it 

 is evident that the position of the right auricle, and its connection 

 with the left and posterior part of the body-wall and foot, is a 

 secondaiy phenomenon, due to its movement from right to left, 

 in the dii-ection of the hands of a clock, during the j^rocess of 

 detorsion. In fact, one can only use the term "right"' in a 

 morphological sense, to indicate that this auricle would be on the 

 right if the typical gasti-opod torsion had been maintained. In 

 the genei"a Nerita and Neritina, as will be seen, the rudimentary 

 auricle is more distinctly on the right side, but even in these forms 

 jDosterior to the ventricle. The size of the right auricle varies 

 much in the Neritidas. It is always present, but in some of the 

 typically marine forms such as Kerita jyeloronta it is so small and 

 unimportant that it might easily be overlooked, and indeed its 

 existence has been denied by Bela Haller. In the tropical 

 Neritinse, many of which are semi-aquatic in habit, spending no 

 inconsiderable part of their lives on tlie roots of trees above low- 

 water mark, or even above high- water mark of neap-tides, the 

 right auricle is larger and receives the same blood-supply as in 

 Septaria. The last-named genus is one that, according to all 

 accounts, has progressed further than any othei" Neiitid in the 

 direction of a terrestrial life, living as it often does on stones in 

 the vicinity of waterfalls where it is only wetted by spray. One 

 might expect, therefore, that it would exhibit a more marked 

 tendency to the replacement of a branchial by apallial respiration,, 

 and such has been shown to be the case. It is of special interest 

 to observe the connection between a more highly developed pallial 

 respiration and the increased size and importance of the right 

 auricle, for, as I shall show in the second part of this memoir 

 dealing with the Helicinidge, there is every reason to believe that 

 in the last-mentioned fainily, in which the ctenidium is lost and 

 the respiration is entirely pallial (or, as it is called, pulmonary), the 

 single auricle that persists is the right and not, as has generally 

 been supposed, the left. 



The courses of the main eftei'ent or arterial vessels proceeding 

 from the ventricle, and the venous sinuses in the foot and visceral 



53* 



