10 G1L15ERT C. BOUUNE. 



the space between the foot and mantle, as shown in fig. 7. 

 The basal portion of the axis is also enlarged at tiiis point, 

 and gill -filaments are given off from both sides, both of the 

 free apex and of the broad basal portion. These filaments 

 are not simply digitiform like those of the left ctenidium, 

 but are plate-like, with the glandular ridge running along 

 their dorsal margins, as shown in fig. 7. As the skeletal 

 bars and glandular ridges are on tlie inner side of the 

 filaments of the inner row, the free axis must have been 

 rotated through 45° to bring the dorsal surface inwards. 

 The plate-like filaments springing from the expanded base of 

 the free part of the axis spread out on, and are attached to, 

 the adjacent parts of the mantle; the filaments, or as they 

 more appropriately might be called, the " gill-lamellae" of the 

 inner row extending dorsal ly along the inner surface of the 

 mantle, while those of the outer row, two or three in number, 

 pass round the front edge of the columellar muscle and run 

 back for some distance below it as ridges projecting inwards 

 from the dependent margin of the mantle (fig. 8, m. h)'.) The 

 blood supply to the ctenidia will be described in connection 

 with the heart. 



Tlie rectum runs diagonally from left to right in the roof of 

 the mantle-cajvity, and the anus opens opposite the slit in the 

 mantle edge. In much contracted specimen.s, such as that 

 from which fig. 2 was drawn, the anus is situated some dis- 

 tance from the slit, but in other less contracted specimens 

 it is close to it. 



The hypobranchial glands lie in the roof of the mantle on 

 either side of the rectum, between it and the ctenidia. Both 

 consist of a more or less extensive modified glandular patch 

 of the internal epithelium of the mantle. The gland-cells are 

 very large relatively to the size of the animal, and are of two 

 kinds : large ovoid cells filled with large granules which 

 stain deeply in hasinatoxylin and green in picro-indigo-car- 

 nnne ; these are therefore mucigeuous cells. The other 

 gland-cells are of nearly the same size and shape, but have 

 clear or minutely granular contents. The left hypobranchial 



