^ 





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^ <.MJtK 







Micron 



0.5 



Figure 130. — Cross section of the group of frontal cilia of the gill of C. virginica. Microvilli of the cell surface are seen 

 at the bottom. Electron micrograph. Buffered osmic acid 1 percent. 



The orientation is apparent in the electron micro- 

 graph (fig. 1.31) of a longitudinal section of the 

 distal part of the lateral cell of the filament of 

 C. virgmica and on transverse sections of the 

 frontal cilia (fig. 130). Because the latter cilia 

 are curved in the direction of the beat, they were 

 cut transversely and appear in the micrograph a 

 short distance above tlie cell surface. Tlieir oval- 

 shaped axial filaments are oriented parallel to the 

 surface of the cell, i.e., in the du'ection of ciliary 

 beat. The membranelike laterofrontal cilia con- 

 sist of several individual cilia embedded in a 

 protoplasmic membrane, but each element retains 

 the typical structure of a single cilium (fig. 131). 

 The basal corpuscles of cilia are arranged in 

 rows (fig. 132), and the central part of each is 

 siu'rounded by denser cortex, giving the appear- 

 ance of an empty central cavity. In the longi- 

 tudinal section (figs. 131 and 132) they are 

 elongated with a pair of rootlets arising from 

 each proximal end. Rootlets of the cilia of the 

 clam, EUiptio complaiiatus, have a periodic stria- 



tion of about 750 A. Similar periodicity appears 

 in electron micrographs of oyster cilia made in 

 the course of my studies, but the pictiu'e is not 

 as clear as that published by Porter and Fawcett 

 (see DeRobertis, Nowinski, and Saez, 19.54, p. 

 382). 



The distribution of rootlets follows a precise 

 pattern. Each rootlet of a pair turns at an acute 

 angle and crosses over the rootlet of the adjacent 

 corpuscle. The rootlets may be followed fiu-ther 

 down the cytoplasm toward the nucleus (not 

 shown in the micrograph) ; some of them cross 

 the second rootlet emerging from the other side 

 of the same corpuscle as can be seen at the center 

 and left side of figure 132. The crossed rootlets 

 are in close contact with each other, but it is 

 not clear whether or not they are fused. Ap- 

 parently direct communication between the basal 

 corpuscles is lacking. 



Tlie cjuestion of whether tiie rootlets are simply 

 the anchoring structures of the cilia or play an 

 active part in its movement remains unanswered. 



THE GILLS 



133 



