BY H. LEIGHTON KESTEVEN. 627 



surface on the axial side, a little distance posterior to the 

 nephridium (fig. 2, c). From here it may, without dissection, 

 be followed along the axis of the coil to the end, giving off 

 branches all the way (fig. 2, c.', c", c.'") ; those given off 

 superiorly are much the largest, and in some cases almost embrace 

 the coil; the finer branches of these reticulate throughout the 

 genital gland and liver. 



There are only two veins, properly so called — the branchial 

 (fig. 2, fig. 5, br.v.) and nephridial (fig. 2, neph.v.); the former is 

 the larger, and enters the auricle almost immediately after leaving 

 the branchiae. The latter, although the smaller, is perhaps not 

 less important; its branches reticulate over, and collect the blood 

 absorbed through the wall of the nephridium, to discharge itself 

 into the branchial vein just outside the pericardium. It seems 

 probable that this vein supplies the new blood to the system. 

 My sections show venous sinuses between the inner and outer 

 wall of the mantle. Their presence was to be anticipated, con- 

 sidering the size of the artery- supplying the right side of the 

 manile. From the ventricle through the right pallial artery, and 

 pallial sinuses to the branchi^, and back to the auricle, is then 

 the shortest circuit. It was this that led me to suggest that the 

 so-called "filiforme prolongations" of some branchiae are really 

 venous sinuses. 



Nervous systeiti (figs. 12, 13) : — The main ganglia and their 

 commissures onl}^ are here described and figured. So far as my 

 dissections have gone, the system is essentially the same as in 

 Littorina. It is hoped in a future paper to describe this part of 

 the anatomy of Risella in more detail, and to compare it with the 

 nervous systems of several of the other genera in the superfamily. 

 The difficulty attendant on getting material for such a comparison 

 has rendered it impossible to incorporate it in the present essay. 

 The cerebral ganglia (figs. 12, 13, e.g.) and their commissure 

 are not at right angles to the antero-posterior axis of the 

 mollusc, but at an angle to it of about 45°, the right ganglion 

 being the more anterior of the two. Anteriorly both ganglia 

 give off three or four nerves, but I have been unable to definitely 



