Pomona College, Claremont, California 59 



Sections were made of the ganglia. The cerebral ganglia were found to be more 

 complex than those of some other bivalves. This was shown in the differences in cell 

 groups and greater complexity of the central fiber masses. The individual cells differ 

 greatly in size, but they average somewhat larger than in some other bivalve forms 

 studied. 



The pedal ganglion, although not so complex, also shows differences between cells. 

 There are large multipolar cells and among these are small ones of various sizes. 

 Tlie processes of the larger cells may be traced into the fibrous mass for some distance. 



The visceral ganglion is composed of two large lateral parts closely fused. 

 There are numerous commissural bands binding the two sides, but the chief fusion 

 is by more or less individual fibers. Cells inclose the whole ganglion and as in the 

 other centers they are of large and small size. The cell areas of the larger cells are 

 mostly localized on the dorsal and upper surfaces, but the lower end of the ganglion 

 has some large cells. The large cells are especially found in the neighborhood of the 

 larger branches, those branches supplying the mantle and siphons and it seems that 

 some of the larger cells are concerned with supplying these characteristic parts. 



The California Piddock, Parapholas californica Conr. 



The ganglia were dissected in medium sized individuals. The cerebral ganglia 

 are about as in other bivalves. The ganglia are quite widely separated. Besides the 

 commissures connecting them and connectives to lower ganglia there are several 

 branches to the mouth region from the upper and lateral sides. 



The visceral ganglion forms a larger mass than any other of the ganglia. There 

 is verv little indication of right and left halves. Closely joined to it is the small pedal 

 ganglion. 



Microscopic examination of serial sections bring out further details. 



The cerebral ganglion is simple in structure. There are a large number of cells 

 in proportion to the fibres in the center of the ganglia. As in many other molluscs, 

 there are many small cells and a few much larger ones, but these last are not abun- 

 dant. In the large cells it is not difficult to determine fine fibrils and strands from the 

 smaller cells near by. There is also a very complex mingling of strands from the 

 central fibrous mass. Some of the fibers are small, some are larger. The appearance 

 of these larger cells is much as described by .'\pathy. The cells in the ganglion are 

 chiefly multipolar. 



The visceral ganglion is the largest and most complicated. Caudally it sends 

 two thick nerves backwards. These are its chief branches for a long distance; they 

 do not branch. The two sides of the ganglion are joined by many cross fibers and 

 there a few bundles in distinct commissures. Most of the cells are small, but there 

 are a few of the larger type. The cells form a rather uniform sheath all about the 

 ganglion, but here and there we find special cell areas. The fibers are much less 

 evenly disposed and present a very complex mat in every part. 



The large cells in some cases have a symmetrical distribution. There are certain 

 individual lateral cells of this sort, also some dorso-central ones which seem to occupy 



