1883.] on Oysters and the Oyster Question. 339 



object, and corresponds in shape with the adductor impression on the 

 shell. That is to say, it has the form of a half oval, the straight side 

 of which looks dorsally and a little forward. The upper and anterior 

 portion of the muscle is darker than the rest and sharply defined 

 from it (Fig. 1, add.). 



Just above the straight side of the muscle, a dark patch indicates 

 the place of the heart, which may be seen pulsating in the chamber 

 or pericardium which contains it (Fig. 1, h). Above this the 

 surface of the body is covered by a smooth and delicate skin or 

 integument, through which, in the breeding season, the reticulated 

 whitish tubes of the reproductive gland shine. 



The chief part of the body of the oyster, which for want of a better 

 name may be termed the trunk, is a somewhat pyriform mass which 

 extends from the ventral contour of the adductor to the posterior half 

 of the dorsal region, and lies much more in the posterior than in the 

 anterior half of the body (Fig. 2 (A) ). 



The rest of the body of the mollusk is chiefly formed by two 

 broad folds of the integument which are given off from the lateral 

 margins of the trunk on each side ; extend backwards, forwards, 

 and downwards ; and, closely applied to the inner surfaces of the 

 valves, end by thickened free margins, which have two rows, an inner 

 and an outer, of close-set papillse. These three folds of the integu- 

 ment are called the lobes of the mantle (Fig. 2 (B), r.mt, l.mt). 

 Their surfaces are attached by a series of delicate muscular fibres to 

 the inner surface of the shell, at some distance from its margin and 

 from their own free edges ; and, in the living state, the fringe, 

 beyond the line of attachment, extends to the edges of the gape and 

 plays the part of a sensory apparatus. The margins of the mantle 

 lobes pass into one another above, at the anterior and posterior ends 

 of the dorsal integument respectively, so that the cleft between them 

 does not extend on to the dorsal surface. The lobes are much deeper 

 in front and below than behind ; hence the cavity which they inclose 

 is correspondingly deeper and shallower in the respective regions. If 

 one lobe is cut through, immediately beneath the anterior end of the 

 dorsal integument, and turned back, it is seen to bound a wide space 

 which extends back a long way, in fact nearly to the posterior side of 

 the trunk. This is the vestibule (Fig. 2 (A), vb\ and the dorsal 

 integument which covers it is the anterior hood or cucuUus (Fig. 2 

 (A), c). Projecting into this is seen a sort of cone which at its 

 upper and front end bears the wide slit-like mouth, bounded by 

 broad lips, one above and one below. The angles of these lips are 

 produced like an upper and lower moustache, into two broad 

 triangular flaps, the so-called labial palps (Fig. 2 (A), p). Below 

 these, the mantle lobes, throughout their anterior and ventral regions, 

 include a wide space termed infra-branchial (Fig. 2 (B), in. br.ch.), 

 because the four plates which constitute the gills or branchice, and 

 are commonly called the " beard," project into it and form its roof. 

 Each of these plates or lamellce is V-shaped in transverse section 



