224 IJARTIN F. WOODWARD. 



in fig. 16, which represents vertical sections across two gill- 

 plates. The ventral and dorsal margins are seen to be 

 dilated^ as also are the blood-spaces nearer the dorsal margin, 

 in which region also the interlaminar connections are larger 

 and fewer, whence the dotted appearance seen in fig. 14. The 

 curious crumpling shown in the ventx*al part of these plates 

 (fig. 16) represents transvei'se sections through the folds seen 

 in the surface view of gill-plate (fig. 14) ; this appearance 

 suggests that the margin of the gill-plate is too short to 

 surround the central area without the latter becoming puck- 

 ered. The gill-plates present another interesting feature in 

 the presence along their outer margins of a couple of sup- 

 porting rods {s. r.), the relations of which are well shown in 

 figs. 15 and 18. From an examination of a transverse section 

 of these rods (fig. 18) it will be seen that they are flattened 

 structures, closely applied to the base of the epidermal cells, 

 and enclosing between them portions of the blood-space of the 

 gill-plate. A section taken parallel to the gill septum and 

 passing through the dorsal junctions of the gill-plates with 

 the septum (fig. 17) shows that the two rods in each plate are 

 perfectly independent of one another, and that each rod is 

 related to two gill-plates. In other words, each rod is a 

 U-shaped structure which embraces the space between two 

 gill-plates, one limb extending into each of these plates, a 

 condition which at first sight reminds one of the relation of 

 the gill-skeleton in the Lamellibranchia. The epithelium 

 covering the margin of the gill-plates is thickened and special- 

 ised; that covering the ventral margin is ciliated (fig. 16), so, 

 too, is that covering the outer border of the plate (fig. 18). 

 Specially long cilia are present near the outer margin in two 

 bands, one on either side of the plate a short distance from 

 the free margin (figs. 16 and 18); the cells bearing these 

 cilia are pai'ticularly large, and are closely related to the 

 supporting rods. 



Comparison with the Lamelli branch Gill. — The 

 general structure and relation of the gill as seen in the 

 zygobranchiate Diotocardia is highly suggestive of that 



