MORPHOLOGY OF LAMELLIBRANCHIATE MOLLUSKS. 
411 
case they have probably become fertilized from spermatozoa taken into the kidney 
from the exterior, having come from another individual, if it is always true that the 
male elements in these hermaphrodite forms are always ripe before the female, as obser- 
vations seem to indicate. The great majority of eggs are of course discharged into 
the water, and there become fertilized and develop. 
It thus happens that the spacious kidney of Pecten becomes a brood pouch for some 
of the youug, and I have seen a kidney containing great numbers of eggs far advanced 
in segmentation, which, it seems probable, both on account of their number and the 
connection between kidney and sexual glands, developed there from the beginning. 
It is true that this arrangement may be a purely accidental one, but I am inclined to 
believe that the advantage gained in protecting so many embryos for so long a period 
in then- development is considerable, and that on this account the kidney may, by the 
operation of natural selection, have become especially adapted for this function. 
THE NERVOUS SYSTEM. 
Of the three pairs of ganglia the cerebral are usually situated near either side of 
the mouth and are joined by a supraoesophageal commissure. They are generally close 
to one another in the more primitive forms. The visceral ganglia, generally placed 
in the ventral side of the posterior adductor, occupy iu Solenomya , Nucula, and a few 
others, a position in front of it. Connected with the pedal ganglia is the otocyst. 
Though it often lies on the surface of the latter, it is probably always innervated from 
the cerebral ganglia. In some cases the otocyst contains a single otolith, in others 
there may be several small particles. 
Sections of the otocyst of Yoldia show an epithelium, the cell walls of which could 
not be distinguished. Nuclei of different shapes and sizes are scattered about irregu- 
larly through it. These cells rest upon a dense supporting membrane, and the whole 
is enveloped in a capsule possessing a fibrous appearance. There is but a single large 
otolith, whose concentric structure is very evident. There is also in the cavity of the 
otocyst what appears to be a coagulated fluid. The lining epithelium, as far as I was 
able to see, possessed no trace of cilia. This apparent absence of cilia has also been 
described for the otocyst of Nucula , by Pelseneer (No. 17). 
THE GILLS. 
The gills, of which there are four, hang in the branchial chamber. They are 
represented in Fig. 96, PI. xciv ( Venus mercenaria), the right valve and mantle lobe 
having been dissected off. The line of attachment of this mantle lobe dorsally, 
beginning at the posterior end at the side of the siphons, is upward along the back of 
the posterior adductor ; thence it proceeds forward in a curved line near the top of 
the pericardium and visceral mass to the anterior foot-retractor, which pierces it, and 
over the anterior adductor. 
The free edge of the left mantle lobe is seen at the point me. An extremely large 
branchial chamber is thus formed, the upper boundary of which is the line of attach- 
ment between the mantle and outer gill. In it, over the walls of the visceral mass, 
hang the gill plates ( og and ig), and below is suspended the foot. The gills of Ostrea 
virginiana (Fig. 97) extend from near the anterior end of the body backward, and up 
for some distance on the posterior side. In Pecten their extent is so great as almost to 
