222 BULLETIN OF THE BUREAU OF FISHERIES. 
genital duct. Their central ends I have not traced distinctly into the nerve to the 
anterior ganglion, but it seems justifiable to suppose that this is their connection. 
What the function of this organ is I can not state. While the figures apparently 
show it some distance from the exterior, it should be remembered that, in ship-worms 
30 mm. long, the sexual duct is less than a half millimeter long, and that the sense 
organ is really very near to and, for purposes of sensation, practically at the surface. 
Osphradia.—In the ship-worm these molluskan organs of special sense form 
large masses of complex tissues at either side of the visceral ganglion (fig. 60). Their 
general shape is elliptical and they are in close association with the branchial nerves. 
Each organ (fig. 65) is composed of two parts. At the ventral (outer) surface there 
is a part of the body epithelium, which in this region is specially differentiated from 
the surrounding cells. While the epithelium of the epibranchial cavity is ciliated, 
the osphradial epithelium is quite devoid of cilia. Besides, the cells composing the 
osphradial epithelium seem to have quite lost their cell walls, so that the spherical 
nuclei lie in acommon mass of protoplasm. The outer surface of the epithelial layer 
is covered by a very delicate membrane, and at its internal surface there is a stouter 
basal membrane. Underlying the surface epithelium is a mass of nervous elements, 
composed of both cells and nerve fibers. The cells, however, are sensory and stain 
somewhat differently from the ordinary ganglion cells. They are of two kinds, both 
spindle-shaped, and sending their peripheral ends through the basement membrane 
of the overlying epithelium, to break up into brushlike terminations just inside the 
delicate outer membrane of the epithelium. These structures are shown in figures 
65 and 66. In figure 65 both types of cells are shown, the larger one to the left rep- 
resenting the type much less numerous than the other, staining differently from 
them, and penetrating the osphradial mass to terminate centrally differently from 
the smaller, more numerous cells. The internal or central connections I have not 
been able to determine, but this much it seems justifiable to state: The osphradial 
nerve from the anterior ganglion becomes so closely associated with the respiratory 
nerve that it can not be stated that the anterior ganglion alone supplies the osphra- 
dium. Also, the large sensory cells penetrate through the osphradial mass, and 
especially it can not be stated that their connection is with the osphradial nerve. 
These structures I have described in some detail for two reasons. In the first 
place, the epithelium of the osphradium is usually described as consisting of colum- 
nar cells, which form the sensory part of the structure. This I have found to 
consist of a layer in which cell outlines are not distinguishable, and in which the 
spherical nuclei lie as in syneytium. The real sensory cells are the spindle-shaped 
cells lying in the deeper part of the osphradium. 
In the second place, Pelseneer (1891) has described the osphradium in ship-worms 
and Pholas as innervated by a nerve from the anterior ganglion, and the latter as 
connected with the cerebral ganglia through the connectives. From this he concludes 
that the osphradia, as well as the other organs of special sense, are innervated from 
the cerebral ganglia. The organization of the nervous system in ship-worms, it 
seems to me, lends no evidence whatever to this view. The nerve fibers received 
from the connectives by the anterior ganglion are quite lost in the latter, and can not 
be traced into any of the nerves which leave it. Moreover, the anterior ganglion 
may, with much more reason, be said to be connected with the visceral ganglion, for 
