ANATOMY OK PELECYPOJJA BURNE. 237 



In connection with these caecal tubes, it is of interest to note that in the three 



species in which thev have been observed (/.<'.. l/isxarrn. Ail<irinir<-n, l'liil,>l>ri/n) there is 

 a, modification of the epithelium along the inner margin of the gill-axis. In this 

 position, start hi; from a point in about the same transverse plane as the pedal ganglia, 

 is an elevated ridge of deep columnar epithelium. At its commencement (Fie;. 7, '/'./'.) 

 the ridge is narrow and sharply defined, resembling closely, when cut transvcrselv, a 

 cross section of an abdominal sense-organ. Further back it gradually becomes broader 

 and Hatter, and finally merges in the epithelium covering the branchial nerve. Without 

 further knowledge one hesitates to suggest what the function of this band of elevated 

 epithelium may be. but the fact, that it occurs, so far as observed, only ill the same 

 species as possess the above-mentioned labial caeca is certainlv of interest, and possiblv 

 may mean that the two organs are in some way functionally related to one another. 



The alimentary canal (Fig. 1) is quite similar to that of other members of the 

 t'amilv (see Matthias, 6, p. 363). The stomach is in wide communication on either 

 side with the lobules of the hepato-pancreas. The latter, as seems to be the case in 

 all the more primitive Lamellibraiichs. are large, simple, and verv looselv packed 

 together. The descending arm of the stomach (gastro-intestine, Fig. 1, </.inf.) is 

 capacious, and closelv resembles in its internal structure that of Area luirlntn, figured 

 by Matthias (6. 1*1. VIII, Figs. 13, 17). The upward bend of the intestine passes 

 below and behind the heart on its way to the upper surface of the adductor. 



Heart and Pi' r tea >'d him. The auricles are uot united behind the ventricle, as 

 they are in some species of Area. The ventricle, although slightly expanded in a 

 transverse direction and bifid at either end, is not markedly drawn out to either side 

 and separated, as in so many members of this family in which, owing to the 

 development of the retractor pedis, the width of the back is excessive. 



The pericardium also (Fig. 1, j>./\), though large, is single; it is produced forward 

 ou either side, forming a pair of funnels leading to the reno-pericardial ducts (Fig. 1, 

 r.jx'.d.}. There is 110 development of pericardial gland-tissue upon the auricles. 



llt'nal Ofifanx. The nephridium is of the simple type characteristic of the family, 

 and resembles in all particulars that described for Adacnarca by Pelseneer (12, p. 4:.'). 



The two organs are quite separate from one another. Each (Fig. 1, /.) is a simple 

 sac covering the anterior face of the adductor, and wrapped round the retractor pedis. 

 The inner surface is for the most part plain, but towards the anterior end of the organ 

 is to some extent pleated. The kidney opens to the exterior close to its anterior end 

 (Fig. 1, i:u.), and slightly behind this point receives ventrally the reiio-pericardial duct 

 (Fig. 1, i:/"'.</.), which passes to this point from the pericardium across its mesial 

 surface. 



Cli'itt fit/iff ()r<i<nix. The sexes are separate, and in the specimens examined the 

 gonads were ripe and active. 



The gonads of the two sides are not completely separate, but. as recorded also bv 

 Pelseneer (12, p. 4.L') in his account of Adacnarca, axe in direct and open connection 



