f.c.t 



ch.a. 







_L 



Microns 



200 



Figure 124. — Longitudinal section near the base of the 

 gill of C. rnrginica. Chitinous arches, ch.a., are em- 

 bedded in fibrous connective tissue, f.c.t. Osmio acid 

 fixation, iron hemato.xylin. 



frontal and laterofrontal cilia of tiie principal 

 filaments have the same structure as those of the 

 ordinary filaments, difTering only in their <>reater 

 size. A short distance below the cell surface each 

 cilium terminates in a basal body, a tiny granule 

 from which a pair of rootlets extends deeper into 

 the protoplasm and becomes undistinguishable 

 near the nucleus (figs. 12(1 and 127). 



^vlj^ 



Microns 



Figure 125. — Transverse section through ordinary fila- 

 ment of C. virginica. Vertical chitinous rods (stippled 

 areas) and blood space are at the center, fr.c. — frontal 

 cilia; If.c. — laterofrontal cilia; I.e. — lateral cilia; o. — 

 ostium. Kahle fixation; hematoxylin-eosin. 



Besides the frontal and laterofrontal cilia, 

 Atkins (1938) distinguishes in 0. edulis the "fine 

 frontal" and "paralaterofrontal" cilia, which run 

 on both sides of the central portion of the frontal 

 tract (fig. 127, f.f.c, para l.f.c). He states (1938, 

 p. 367) that: "Subsidiary laterofrontal cilia are 

 present in Ostreidae, but are very difficult to 

 distinguish even in the living gill." I was unable 

 to identify these cells in sectioned preparations or 

 in the living gill filaments of C. virginica examined 

 under high power. The frontal cilia of this 

 species appeared to be of uniform length along 

 the entire cross section of the tract (fig. 125). 



Beneath the laterofrontal cilia of the filaments 

 there is a group of six large cells, four of them 

 broad and two narrow, which bear large, stout 

 cilia about 17 to 18 m in length. These are the 

 lateral cilia (fig. 125 and 127, I.e.), which bend 

 forward slightly toward the outer surface of the 

 filament and touch the ciha of the opposite group. 



TERMINAL GROOVE 



The free edge of a demibranch formed by the 

 concrescence of the ascending and descending 

 lamellae is a shallow trough called the terminal 



THE GILLS 



129 



