| OCR Text |
Show 1 895.] ANATOMY OF NAUTILUS POMPILIUS. 677 much in form-sometimes being divided into two distinct halves- sometimes continuous mesially-sometimes thin and membranous- sometimes tumid and swollen. It is covered by columnar epithelium, and filled with ordinary connective tissue, sometimes with abundant jelly-like matrix. In the female, examination of this region with a hand-lens shows the existence of a large number of apertures in the outer skin. These, to the number of about 150, form a band about 0-5 m m . in width, curving gently forwards on either side of the post-anal papilla. tapering off and terminating close to the advehent vessel of the posterior gill. In section these openings are seen to be the apertures of tubular ducts which pass inwards perpendicular to the surface for some little distance 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 cilia at the same time becoming extremely long and powerful (*03 m m . in length). Once within the narrow aperture the lumen of the tube expands to about •05 m m . 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-cesophageal 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 neiwes 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 b e - although the condition of the material did not allow m e to quite satisfy myself on this point-an anastomosis of these centrally passing branches with those of the opposite side. If this be confirmed w e have here a true post-anal commissure, such as exists in Chiton: in which case w e should be compelled to regard not merely the " posterior sub-cesophageal 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-cesophageal nerve-mass would then represent a secondary fusion between the nerve-masses of the two opposite sides. VII. The Spermatophore-receiving Apparatus, Behind (ventral to) the buccal mass and immediately underlying 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 lamellae. The remarkable organ formed has been referred to by Valen- |
