THE ANATOMY OF PLEUROTOMARIA BETRICHII. 231 



in other Prosobrancliia a fairly thick investment to the dorsal 

 surface of the liver (fig. 1, g. g.), extending to the top of the 

 spii'B. 



The origin of the efferent duct from the gland was not to 

 be made out owing to the upper part of the body being slightly 

 damaged in i-emoval from the shelly but I think there can be 

 no doubt that the genital products are shed into a series of 

 thin-walled remnants of the true coelom, which in turn unite 

 to form the somewhat thickened duct {g. d.) shown in figs. 

 23 — 26. This duct, which is present in both sexes, runs on 

 the inner side of the spire, and communicates by a slit-like 

 opening {g.a.) with that portion of the right kidney-chamber 

 which we termed the ureter («.). The conclusion that this 

 is the genital duct is supported by a comparison with 

 Trochus, where the undoubted genital duct has precisely 

 the same relationships. 



In the male this constitutes the whole of the genital system, 

 there being no accessory organs, the genital products passing 

 out directly through the unmodified right ureter. In the 

 female, however, the distal portion of the ureter which serves 

 to transmit both the excretory and genital products becomes 

 much modified, owing to the enormous development of glan- 

 dular tissue in its walls; the latter become so much thickened 

 that it is by no means easy to find the lumen of this tube, 

 which may now be called the oviduct [ov. d.). 



The presence of this modified oviduct places Pleuroto- 

 maria about on the level with the TrochidaB, and indicates a 

 somewhat more specialised condition than that met with in 

 many Diotocardia, for in these latter the genital products are 

 discharged into the mantle-cavity through the unmodified 

 right kidney duct, — in some cases, it is thoug-ht, without the 

 intervention even of the simple genital duct seen above. 



The Alimentary Canal. — The mouth communicates with 

 a thick-walled buccal cavity situated in the free portion of 

 the head. This buccal mass, which is slightly constricted by 

 the nerve- ring, is closely attached to the body-wall by nu- 

 merous short radiating muscle-fibres (figs. G, 7, and 8), which 



