830 PaOF. G. C. BOURNE ON THE [iSTov. 17, 



base attached to the body-wall on the left side. It consists of a 

 flattened axial plate containing blood-spaces, with numerous 

 gill-lamellse running transversely across its dorsal and ventral 

 surfaces. The ctenidium is therefore typically bipectinate. 

 Unlike that of N.fluviatilis its posterior half is attached, on the 

 left side to the mantle close to its union with the columellar 

 miTScle, on the right side to the lower surface of the kidney, by 

 a membranous suspensory fold. Thus the posterior part of the 

 mantle-cavity is divided into an upper and a lower chamber. A 

 large blood-vessel runs along each edge of the axial plate, and in 

 the upper and lower wall of each vessel there is a stout band of 

 longitudinal muscle-fibi'es, which must serve as retractors of the 

 gill and also assist in the cii'culation of blood through the gill- 

 lamella?. On the right-hand is the afferent and on the left the 

 efferent branchial venous sinus. The two do not communicate 

 with one another at the apex of the gill but only by the lacunar 

 passages in the gill-lamelke and the axial plate. The general 

 course of circulation in the gill is as follows : — Blood is brought 

 to the gills from the large venous sinus underlying the kidney 

 by the afferent branchial sinus. This sinus does not com- 

 municate, as may be seen by inspection of fig. 20, with the 

 cavities of the axial plate, except at very rare intervals, but it is 

 in free communication above and below with the cavities of the 

 gill-lamellfe, and its blood passes into these latter and circulates 

 through them. The cavities of the gill-lamella? and also that of 

 the axial plate are broken up by numerous trabecular or partitions 

 passing from wall to wall and are bounded by a thin layer of 

 connective tissue containing a few muscular fibres. The margin 

 of each gill-lamella is somewhat swollen and contains a cavity or 

 vessel which, as far as I can ascertain, is continuous from one 

 end of the lamella to the other, and also is in free communication 

 with the irregular sjmces below. At the opposite side of the gill 

 these marginal vessels open into the efferent branchial sinus. 

 The innermost cavities of the gill-lamella? o-^en from place to 

 place into the system of lacunfe in the axial pla,te, and these, 

 uniting to form a lai-ge lacuna near the left edge of the axial 

 plate, also open at frequent intervals by large apertures into the 

 efferent branchial sinus. It is obvious that the blood on entering 

 the gill-lamellfe from the afferent sinus may either take a direct 

 course to the efferent sinus by way of the marginal vessels of the 

 lamella?, or may traverse the lacun?e in the adaxial part of 

 the lamellse, pass into the system of lacunae in the axial plate, 

 and thence be discharged into the efferent sinus. 



The distribution of ciliated epithelium on the faces of the gill- 

 lam ellse is of some interest. Each gill-lamella is an extremely 

 delicate plate of semilunar form, its straight inner margin 

 attached to the axial plate, its curved margin fi-ee and, as 

 described above, somewhat thickened. When one attempts to 

 separate the lam elite by the aid of needles one recognizes that 

 their central portions adhere very closely together, whereas their 



