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 young, 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 tliat 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 their 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 in Soleriomya, Wucula, 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 au epithelium, the cell walls of which could 

 not be distinguished. Nuclei of diiferent 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 apj)ears 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 oi Nucida, 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 rigbt valve and mantle lobe 

 having been dissected off. The line of attachment of this mantle lobe dorsaUy, 

 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 tlie anterior adductor. 



The free edge of the left mantle lobe is seen at the point wie. 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 

 virginiona (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 



