34 



Our Food Mollusks 



interior chamber of the gill. They contain blood and 

 are probably developed to facilitate its oxygenation. 



While the oyster has become, through degeneration, 

 one of the simplest of lamellibranchs in general struc- 

 ture, its gills are among the most highly modified and 

 perfected to be found in the group. In a general way, 

 this may be explained by saying that the extensive de- 

 generation of certain organs has resulted from the habit 



FIG. 7. The gill of the oyster (Ostrea virginica). Reference- 

 letters b s indicate blood spaces ; /' and /" enlarged fila- 

 ments at reentering angles of lamellar folds; other letters 

 as before. 



of fixation, and that this mode of life has made special 

 demands on the gills, which have been rendered more 

 complex in consequence. 



Comparing the diagram of the oyster gill (Figure 7) 

 with the others, the folding of the lamellae seen in the 

 scallop gill, is developed to the greatest degree. Be- 

 tween folds as in Pecten, a single filament is greatly 

 enlarged (/'), and at regular intervals a large interlam- 

 ellar partition appears, uniting the inner edges across 

 the gill chamber (/"). As in the little neck gill also, 



