MINUTE STEUOTURE OF GILLS OF PAL^MONETES VARIANS. 79 



nuclei in it has not been made out, owing to the extreme 

 thinness of the layer. That a membrane does actually exist 

 can be seen without doubt; both in transverse and longitudinal 

 sections of the channels. 



The blood passing down the inner border finds its way across 

 the lamella through an irregular network of channels; which 

 lie close to the surface on both the upper and under faces of 

 the lamella. These channels are seen in section in figs. 5; 6, 

 and 7; s. ch. They are lined on their outer side by the ex- 

 tremely thin arching processes of the transverse cells; on their 

 inner by the central cells of the lamella. Fig. 8, which is 

 drawn from a very oblique section of a lamella, shows this 

 irregular network of channels running across its two surfaces. 

 a. I. and v. I. are the two channels running along the borders 

 of the lamella; whilst s. ch. is the network of surface channels 

 running across. 



Owing to the irregularity of arrangement of the transverse 

 cells and of the surface channels; the appearance presented 

 by longitudinal and transverse sections of the central part 

 of the lamella is practically the same (cf. fig. 5 with figs. 6 

 and 7). 



A slight thickening of the lamella occurs along its distal 

 border; and here the surface channels are very much enlarged 

 (fig. 7, d. b.). It is probable that all the blood-corpuscles are 

 carried round through these enlarged channels; as the others 

 have never been seen to contain a corpuscle, and appear to be 

 too thin to admit one. 



The Gill Glands. — In addition to the excretory cells sur- 

 rounding the venous channels a large number of glandular 

 bodies occur in the axis of the gill. These glands (figs. 1 and 

 2; rt. gl. and c. gl.) are spherical in shape, and are composed 

 of large conical cells, each cell having its apex directed towards 

 a common centre. The glands are of two kiuds : in the one 

 (figs. 1, 2, &c., rt. gl; figs. 9, 11, and 12) the body of each 

 cell appears as a deeply staining network ; in the other (figs. I, 

 2, &c., c. gl. ; fig. 13) it is only faintly stained, and shows a 

 more granular structure in preparations preserved in Flem- 



