832 PROF. G. C. BOURNE OK THE [!Nov. 17., 



arrangement of the cells in Septaria, I should have overlooked it. 

 As it is I have identified a similar gill-structuie in Xerita plicata, 

 X. variegata, Neritina gagates, antl X. longisjnnosa. The de- 

 scription given above is therefore characteristic for the ctenidium 

 of the Neritid?e, and will not be repeated in dealing with other 

 species. 



The branchial nerve, derived from the ganglion undei'lying the 

 osphradium, runs through the suspensory membrane and along 

 the outside of the efferent branchial vessel in the free moiety of the 

 gill. This nerve has been noted by nearly all previous authors, but. 

 only Bernard has described a nerve running down ihe other side of 

 the gill, in connection with the afferent branchial vessel. Such 

 a nerve, embedded in the dorsal longitudinal muscle, is present in 

 Hep)taria and is shown in fig. 20, hr.n. As far as I can determine 

 from my sections it communicates with the nerve on the efferent 

 side by a slender connection at the tip of the gill. It is thickest 

 in the posterior part of the course, and passes into the right- 

 suspensory membrane in the direction of the visceral ganglion ; 

 but try as I might, I could not ti-ace it through the suspensory 

 membi'ane to the ganglion. 



The Circulatory System. 



Lenssen's account of the heart and blood-vessels in i\"'._y?^sv^ai^7is 

 is in all essential features applicable to Septaria. I need lay stress. 

 on a few points only. The ventricle in Septaria is thick and 

 muscular and is completely wrapped round the rectum. I wish 

 to emphasize this point, because some authors have stated that the. 

 ventricle is only partially attached to the rectum in ISTeritid^e, and 

 have contrasted this with the conditions found in Haliotidte and. 

 Trochidse. Practically the supposed difference does not exist. 



The blood from the efferent venous sinus passes directly into the 

 left auricle, and at the j^oint where the efierent sinus joins the 

 auricle a large pallial sinus, biinging back blood from the numerous 

 lacunpe of the roof of the mantle-cavity, opens into it. This 

 pallial sinus has been recoirled by Lenssen in X. jiuviatilis, but I 

 think he undeixates its importance. In Septaria^ at anj- rate, its 

 diameter is nearly equal to that of the efferent branchial sinus, 

 and it is connected with so extensive a system of blood-lacunae in 

 the mantle that there can be no doubt that the latter is a veiy 

 important auxiliary organ of respiration. The blood, therefore, 

 which reaches the ventricle by way of the left auricle has been 

 oxygenated either during its passage through the ctenidium or 

 through the mantle, but none of the pallial blood passes through 

 the ctenidium. The importance of the pallial circulation in 

 Septaria is further indicated by the relatively considerable size of 

 the right auricle. This organ, the relations of which may easily be 

 traced in sections or by simj)le dissection, passes from that pai't of 

 the ventricle lying posterior to the rectum, across the pericardial 

 cavity, and is attached to the left body- wall just at the level of the. 



