1887.] MICROSCOPICAL JOURNAL. 63 
whole structure having somewhat the appearance of a short club. They are 
whitish in color, and each bears on the side turned toward the gill a large 
black pigment spot. 
Examination by means of sections shows plainly the central axis in which 
may be seen the cross section of the olfactory nerve lying beneath the outer 
layer and surrounded by the mesoderm. In this respect it is like the struc- 
ture of the osphradium of /ulgur carica.* This trunk is seen to be made 
up of a central portion consisting wholly of fibres which run longitudinally, 
PLATE 2.—Osphradium of Crepidula. 
br. Gill supporting rod; by. Branchial vein; ep. Epithelium of the general surface; m, ep. The modified 
epithelium at the back of the gill ridge; n. The osphradial nerve; p. The osphradial papilla. 
Fig. 1.—View of dorsal aspect of Crepidula fornicata, the mantle cavity diagrammatic, to show position of gill 
and osphradium. 
Fig. 2.—Section across the mantle roof between two papille. 
Fig. 3.—Section across the mantle roof passing through one papilla. 
Fig. 4.—Section of one papilla. 
Fig. 5.—Oblique section across the roof, showing parts of several papillae and the area of modified epithelium. 
invested peripherally by a mass of ganglion cells, whose nuclei are usually 
plainly seen when stained by the action of borax carmine. The whole is 
bound in by an extremely delicate layer of conn. tissue—its proper sheath. 
At each papilla a branch is sent off from the main trunk ; this runs up into 
the papilla through the centre of the peduncle, and appears to spread out and 
become lost in the expanded terminal portion. The papilla consists of a core 
mesoderm and the nerve branch covered in with a layer of ectoderm. The 
ectoderm cells upon the peduncle and lower portion of the tip are shut off 
from the mesoderm by a well-defined basement membrane, and are columnar 
non-ciliated cells. Those upon the side toward the gill further contain 
masses of black pigment. This pigment is so much like that found variously 
scattered through the mantle that it is not probable that its presence is 
connected with the operation of the papilla as a sense organ. Upon the 
outer end of the papilla the cells are very tall and their outlines very ill-de- 
fined. The basement membrane, which is very evident over the sides of the 
+ papilla and over the mantle generally, becomes undistinguishable at this point, 
* See Studies from Biol. Lab. J. H. U., vol. iii, 
