ON THE ANATOMY OF NAUTILUS POMPILIUS. 199 



and then break up into several blindly ending branches. 

 These are lined by involutions of the surface epithelium, which 

 in the neighbourhood of each aperture increases to about twice 

 its thickness elsewhere, its celia at the same time becoming 

 extremely long and powerful (-03 mm. in length). Once within 

 the narrow aperture the lumen of the tube expands to about 

 •05 mm. in diameter, and the lining epithelium becomes shorter, 

 the remainder of the lumen being lined by comparatively short 

 columnar cells, each with a round ellipsoidal nucleus. 



Arising from the posterior side of the posterior sub-oesopha- 

 geal nerve-cord, close to the middle line, are a pair of stout 

 nerve-trunks, which pass backwards on either side of the vena 

 cava. The greater part of these pass off to supply the gills, but 

 a direct prolongation of each is present, which passes backwards 

 on either side of the post-anal papilla. This nerve is largest in 

 the female, where it gives off nerves towards the middle line, 

 supplying the nidamental gland. In the region of the post-anal 

 papilla branches also pass off towards the middle line. There 

 appears to be — although the condition of the material did not 

 allow me to quite satisfy myself on this point — an anastomosis 

 of these centrally passing branches with those of the opposite 

 side. If this be confirmed we have here a true post-anal com- 

 missure, such as exists in Chiton : in which case we should be 

 compelled to regard not merely the "posterior sub-oesophageal 

 nerve-mass," but rather the two lateral portions of this, together 

 with the nerve- trunks which have been mentioned as passing 

 backwards on either side of the vena cava, as forming the 

 homologue of the pleuro-visceral nerve-cord of Chiton. The 

 mesial part of the posterior sub-oesophageal nerve-mass would 

 then represent a secondary fusion between the nerve-masses of 

 the two opposite sides. 



VII. The Spermatophoke-keceiving Apparatus. 



Behind (ventral to) the buccal mass and immediately under- 

 lying the inner inferior lobe, the kind of shelf which connects 

 the tentacular mass of one side with that of the other has its 

 inner surface raised into a series of curious lamelliB. 



