J 895.] A3IATOMT OF IN^ATJTILTrS POMPU/ITTS. 677 



mucb in form — sometimes being divided iato two distinct halves — 

 sometimes continuous mesially — sometimes thin and membranous — 

 sometimes tumid and swollen. It is covered by columnar epithe- 

 lium, and filled with ordinary connective tissue, sometimes with 

 abundant jelly-Uke matrix. 



In the female, examination of this region with a hand-lens shows 

 the existence of a large nimi her of apertures in the outer skin. 

 These, to the number of about 150, form a band about 0*5 mm. 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 giU. 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 ('OS 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-oesophageal 

 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 A\ith those of the opposite side. If this be con- 

 firmed we have here a true post-anal commissure, such as exists in 

 Chiton : in which case we 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-oesophageal 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- 



