



Microns 



200 



Figure 129. — Longitudinal section through the base of 

 a demibranch of C. virginica. Kahle, Mallory triple 

 stain, ex. — e.xtensor muscles; f. — flexor muscles. Pieces 

 of the skeleton arch are shown in black. 



CILIATED CELLS 



The structure and functioti of vibratile elements 

 of tlie cells have been the object of numerous inves- 

 tigations beyond the scope of this book. The 

 reader is, therefore, referred to comprehensive re- 

 views of the problem of ciliary motion made by 

 Gray (1928) and more recently by Atkins (1938) 

 and Brown (1950). Several theories based on 

 studies of the structure and action of cilia fail to 

 give a satisfactory e.xplanation of ciliary motion, 

 which at present still remains a biological mystery. 



Cilia examined in transmitted light or viewed on 

 a dark background in reflected light appear to be 

 optically homogenous. In polarized light they 

 are birefringent (Schmidt, 1937). Observations 

 with the light microscope disclose the presence of 

 an axial filament (axoneme) surrounded by a thin 

 sheet of cytoplasm (^\enyon, 1926). As a rule, 

 the cilia emerge from tiny basal granules 

 near the cell surface and penetrate through the 

 cuticle, which under the light microscope appears 

 as a thin homogenous membrane. Studies of 

 the role and origin of basal bodies in various 

 ciliated cells have resulted in a great number 

 of speculations. Experiments by Peter (1899) 

 showed that in small fragments of a crushed 

 protozoan the cilia continued to beat as long 

 as they were in organic connection with the 

 adjacent pieces of cytoplasm. He deduced from 

 tiiis observation that the ciliary mechanism is 

 located near the surface of the cell. Similar results 

 were obtained with lateral cells stripped away 



from the filaments of Mytilus gills. The cilia 

 that were removed from the basal granules re- 

 mained motionless while those connected with 

 them continued to beat (Gray, 1928). The 

 microdissection technique in more recent years 

 supports these findings. It was demonstrated 

 that in the ciliated cells of the gills of Anodonta 

 the motion of the cilia ceases when the cell is cut 

 transversely in the immediate region of the basal 

 granules. Transverse cuts made at any level 

 within tiie proximal two-thirds of the cell had no 

 efi'ect on ciliary motion, but if the cut was made 

 across the zone occupied by the fibrillae or rootlets 

 in the distal third of the cell, the coordination of 

 the ciliary motion was destroyed although the 

 cilia continued to beat. These observations 

 seem to support the validity of the theory, ad- 

 vanced hidependently by Henneguy (1897) and 

 Lenhossek (1898), that the basal granule, homol- 

 ogous and sometimes identical with the centro- 

 some of the mitotic figure, is the center which 

 controls the activity of the cilium. 



FINE STRUCTURE OF THE CILIA 



With the advance of electron microscopy con- 

 siderable progress has been made in the study of 

 the fine structure of cilia. It has been discovered 

 that throughout the plant and animal kingdoms, 

 regardless of the position of the organism on the 

 evolutionary level and irrespective of the organs 

 studied, cilia have a common structural pattern. 

 The cilia of the gill epithelium of the oyster are 

 no exception to this rule. Thin sections of the 

 frontal and lateral cells of the filaments fixed in 

 buffered osmic acid and examined under the elec- 

 tron microscope show a structure which is undis- 

 tinguishable from that of the cilia of vertebrates, 

 protozoa, or the tails of spermatozoa. The cilium 

 consists of a protoplasmic matrix in which are 

 embedded 11 filaments; 2 single filaments are at 

 the center and 9 double ones are arranged in a 

 ring on the periphery. The central pair is con- 

 nected to the peripheral ring by radial trabeculae 

 or spokes. Short pieces of dense ntaterial join 

 the outer filaments to the membrane (fig. 130), 

 wliicli binds more osmium and is, therefore, darker 

 tlian their interior, making the cilia appear tubular 

 (Fawcett, 1958). The two central filaments are 

 o\al shaped in cross section. The plane in which 

 these filaments are oriented is similar for all the 

 cilia of the cell and is thought to be perpendicular 

 to the direction of the ciliary beat (Fawcett, 1958). 



132 



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