THE ANATOMY OF PLBtJKOTOMARlA BEYBICHII. 223 



form of triangular plates, whose surfaces bear a number of 

 fine plications (fig. 14). In each gill the inner or under set 

 of plates are somewhat smaller than the outer set, a con- 

 dition leading towards the more specialised one seen in the 

 Trochidae. As in other Scutibi'anchs, the anterior end of 

 each gill is not attached to the mantle, but projects freely 

 into the mantle-cavity, and, in the contracted state of the 

 mantle, almost beyond the anterior margin of that fold. 



Structure of the Gill-plates. — A careful study of 

 sections of the gill taken through the three principal planes, 

 i. e. transverse to the long axis, longitudinal sections parallel 

 to the gill-septum, and horizontal sections, enables us to 

 construct a diagram (fig. 15) showing the circulation of the 

 blood in the gill-plates. The afferent branchial vessel (a. h.), 

 as we have already seen, is situated at the ventral edge of 

 the gill-septum under a thickened ridge of glandular epi- 

 dermis ; this vessel gives off on either side small branches, 

 which enter one into each of the very thin gill-plates. The 

 vessel then spreads out as a delicate film between the two 

 laminae which together constitute the plate. After the blood 

 is aerated by being brought into such close proximity to the 

 sea- water it leaves the plate near its dorsal attachment, this 

 efferent channel joins across the septum with the corre- 

 sponding vein from the opposite plate, the conjoint vessel runs 

 up the septum and enters the efferent branchial vessel (e. b.), 

 which lies at the junction of the septum and mantle. It must 

 not, however, be supposed that the space between the two 

 laminae of a gill-plate is as simple as represented diagrammati- 

 cally on the left of fig. 15; such is not the case, the space 

 being broken up into numerous small channels by the pre- 

 sence of great numbers of interlaminar connections {i. I. c, 

 figs. 15, 16, and 18), extending across the space and joining 

 the two laminae which compose the gill-plate. The blood thus 

 takes a very sinuous course among these connections. A 

 somewhat larger channel is, however, present all round the 

 margin of the gill-plate. 



The extremely delicate nature of the gill-plates is well seen 



