228 MARTIN F. WOODWARD. 



vessel lies near the free margin of the gill-septum, while 

 the efferent vessel is situated at its base. 



The Hypobranchial Mucous Glands. — One of the 

 most striking features in the mantle- cavity is a large oval 

 glandular structure, which, occupying a median position, 

 extends from the posterior limit of the mantle-slit along the 

 roof of the mantle-cavity to about the level of the posterior 

 end of the right gill (figs. 5, 6, and 7, m. g.). 



In one specimen this gland still retained a pinkish colora- 

 tion. This gland is partially divided by a median longitudinal 

 furrow into two halves, each of which is marked by a number 

 of more or less interrupted grooves which converge on the 

 median one. The whole structure presents an appearance 

 not uulike the venation of a leaf. Anteriorly, however, the 

 two halves of the gland slightly separate from one another, 

 and end in a couple of pointed structures, in which Dall 

 thought he could perceive openings which he took to be the 

 renal apertures. In this supposition he was mistaken, for 

 the renal organs have a perfectly normal position, and the 

 gland, as may be seen from a microscopic examination, is a 

 true mucous gland : furtlier, if we examine Haliotis we 

 shall find a gland, the hypobranchial mucous gland, occu- 

 pying a precisely similar position ; and I think there can 

 be no question as to the homology of these two structures 

 and of the similarly named gland of the Monotocardia. 



Two additional mucous glands are found in the roof of the 

 mantle-cavity behind the large hypobranchial gland (figs. 5 

 and 7, mg'. and mg" .) ; these are situated one at the base of 

 each gill, that on the left being much the largest, a further 

 example of the asymmetry which we have already seen fore- 

 shadowed in the gills, and which affects the whole pallial 

 complex. 



The rectum is situated somewhat to the right side, and 

 extends forward over the hypobranchial gland in a variable 

 manner (cf. figs. 5 and 7) ; but in neither of my specimens 

 does it extend so far forward as in the P. Adansoniana 

 figured by Dall. 



