200 



Microns 



Figure 122. — Side view (A) and front view (B) of a 

 skeleton of ordinary gill filaments of C. virginica. Soft 

 tissues removed by sodium hydroxide. Unstained 

 preparation, whole mount. 



the ostia are approximately only one-third the 

 dimensions of those in 0. lurida. 



CILIARY TRACTS 



The surface of the filament is covered by 

 several different ciliary tracts. Cells of uniform 

 size on the outer surface bear the frontal cilia, 

 which are relatively small and beat parallel to the 

 surface of the filament (fig. 125, fr.c). They are 

 flanked on each side by a single laterofrontal cell 

 (If.c.) of larger size with a blade-shaped cilium, 

 which according to Atkins (1938) occurs in the 

 family Ostreidae and is somewhat different from 

 the laterofrontal cells of other bivalves. In 

 fixed and stained preparations this wide and 

 curved cilium is frequently frayed. The shape 

 of the cilium and the presence of the basal granules, 

 typical for normal ciliated cells, both indicate 

 that the laterofrontal cilium is formed by the 

 fusion of ordinary filaments, a view which is con- 

 firmed by studies made with the electron micro- 

 scope (see p. 132). The laterofrontal cells of C. 

 virginica are fairly large and cone-shaped; their 

 relatively small nuclei are located at the narrow, 

 proximal end of the cell; and the protoplasm is 

 devoid of granules and deeply stained with 

 hematoxylin. The laterofrontal cilia of gills 

 preserved in a relaxed state extend toward their 

 opposing numbers on the adjacent filament and 

 touch their tips. In a contracted gill they are 

 bent and almost undistinguishable from the 

 cila of the frontal cells. The length of the latero- 

 frontal cila on sectioned and stained preparation 

 varies from 11 to 15 m. Accurate measurements, 

 however, are difficult because of the bending of 

 the cilia wliich do not remain fully extended even 

 in completely narcotized cells. In O. edulis, 

 according to Atkins (1938), tlie length of the 

 laterofrontal cilia varies from 14 to 25 ^i. The 







Millimeters 



0.5 



Figure 123. — Chitinous rod of the principal filament of C. virginica. Soft tissues removed by sodium hydroxide. Un- 

 stained preparation, whole mount. 



128 



FISH AND WILDLIFE SERVICE 



