78 RICE. [VOL. II. 



Cardium cdnle would measure, if flattened out, fully seven times 

 the length of the free margin of the same gill. The absence of 

 fusion in filamentous gills, even where the folding is extreme, as 

 in Pecten, may be easily explained on the ground of the looser 

 structure of the gill and the possibility of a displacement of the 

 filaments, with consequent relief of pressure. 



It is an interesting point in this connection that no fusion 

 was observed in the outer gill of either Psammobia or Cardium, 

 although it is conspicuous in the inner gill. In both cases the 

 inner gill is provided with a deep marginal groove, while the 

 outer gill shows no sign of this structure. The presence of this 

 marginal groove is the mechanical equivalent of a shortening 

 (slight to be sure) of the margin of the gill, the bottom of this 

 groove being almost perfectly straight, and therefore slightly 

 shorter than the somewhat scalloped margin of the folded gills 

 in which no groove is present. Hence we find in this charac- 

 teristic a cause of increased crowding and increased fusion. 



For a very short distance above each point of fusion one 

 observes a slight modification of the epithelium of those sides 

 of the fusing filaments which are turned toward each other. 

 The nuclei are somewhat larger and more closely crowded, as 

 shown diagrammatically in Fig. 5. Aside from this, no histo- 

 logical distinction can be drawn between the simple original 

 filaments of the upper portion of the gill and the compound 

 filaments of the lower margin. Even in the matter of size there 

 is no noticeable difference except in the immediate vicinity 

 of the fusion, where the compound filament is considerably 

 enlarged. This statement is made after comparison of a large 

 number of preparations, and seems to be the rule, although 

 there are considerable individual variations in different speci- 

 mens. The apparent larger size of the compound filaments in 

 Fig. 2 is to be explained in part on the ground of such indi- 

 vidual variations. It is due in larger degree, however, to slight 

 differences in the position of the filaments, which are elliptical 

 in cross-section, and appear of different size according as one 

 or another side is presented to view. It should also be noted that 

 the filaments are somewhat enlarged at the extreme margin of 

 the gill, and that these enlarged tips are shown in the figure. 



