Micron 



Figure 132. — Section perpendicular to the surface of frontal cilia of the filament of the gills of C. virginica. The curved 

 frontal cilia are cross sectioned. Note the row of basal corpuscles with rootlets; the sharp line, parallel to the surface 

 of the cell corresponding to plasma membrane; and the microvilli above it. Electron micrograph. Osmium fixa- 

 tion. Buffered osmic acid 1 percent. 



show that both the contraction of the muscles 

 and the movement of bactei'ial ciha is stimulated 

 by adenosinetriphosphate (DeRobertis, Nowin- 

 ski, and Saez, 1954, pp. 389). 



MECHANICAL PROPERTIES OF THE 

 CILIUM 



Most of the observations on tlie structure and 

 and movements of lamellibranch cilia were made 

 on the gills of Mytilus. There is no reason to 

 tliink, however, that the cilia of the oyster oill 

 are fundamentally different from those of the 

 mussel. 



The gifl cilium is a flexible and elastic rod wliich 

 can be deformed by mechanical pressure applied 

 with a microdissection needle. Tiie deformity is 

 repaired rapidly wlien the pressure is removed. 

 Gray (1928) interprets these observations on 

 Mytilus cilia, as an evidence of transverse elas- 

 ticitv of the cilium. 



Tlie movement of the cilium consists of two 

 distinct phases, the forward effective stroke and 

 the much slower recovery stroke which brings 

 the cilium to its initial position. The velocity of 

 the eft'ective stroke is considered to be five times 

 that of tiie recovery stroke (Kraft, 1890). Tlie 

 eft'ective stroke begins with the curving at the 

 tip and extends down to the base, bending the 

 entire cilium into an arch of 180°; throughout this 

 period the cilium behaves as a rigid rod mounted 

 to the cell by its end. During the recovery 

 stroke the cilium straightens from tlie base to 

 the tip and moves backward as a limp thread. 

 Botii t!ie effective and the recovery strokes take 

 place in tlie same plane, which remains constant 

 (Gray, 1922a; Carter, 1924). 



The movement of a cilium results from con- 

 traction of its filaments. It is not clear, however, 

 wliether all 11 filaments are eciually involved in 

 the effective and recovery strokes. Furtliermore, 

 it appears probable, although definite proof is 



THE GILLS 



135 



