I* 





i, 



* ." >■* 



^f 



0~ 0.5 ' 



J.' ^ Micron 



Figure 131. — Longitudinal section of the distal portion of laterofrontal cell of the gill of C. virginica. Since the plane 

 of section passes at the middle of the cilium only single axial and two peripheral filaments can be seen. The basal cor- 

 puscle and the beginning of rootlets are at the lower part of the micrograph. Electron micrograph. Buffered osmic 

 acid 1 percent. 



There is the possibility that they may represent a 

 coordinating mechanism of the cihary epithehum. 

 The fact that the rootlets of the two adjacent 

 corpuscles cross each other is in favor of this 

 view, which was advanced by Grave and Schmitt 

 (1925) on the basis of their observation of the 

 cilia of fresh-water mussels made with the light 

 microscope. Exploration with the electron micro- 

 scope gives additional support to their hypotliesis 

 which, however, requires further corroboration. 



The free surface of the ciliated cell appears as 

 a thin homogenous layer, devoid of visible struc- 

 ture, when examined in the light microscope. In 

 reality this layer consists of fingerlike processes 

 called microvilli (figs. 130 and 132), which are 

 found in various tissues; they are considered a 

 device to increase the surface of the cell. Their 

 number has been estimated as high as 3,000 per 

 single cell of intestinal mucosa, and there is no 

 doubt that numerous fingerlike processes greatly 

 increase the surface area of the eill and facilitate 



the exchange of gases and ions. In figure 132 

 the layer of microvilli, about 0.5 /j in thickness, 

 rests upon the plasma membrane of the cells. 

 The cytoplasm under the membrane contains 

 numerous mitochondria. 



The complex ultrastructure of the ciliated cell 

 of the oyster gill is shown diagrammatically in 

 figure 133, which represents a reconstruction of 

 the principal features seen on electron micro- 

 graphs. The diagram is based on a large number 

 of micrographs and summarizes our present 

 knowledge of the dimensions and arrangement of 

 the various parts which comprise the ciliated 

 apparatus of the oyster gill. 



Although the mechanism of ciliar_v motion is 

 not known, studies of tlie ultrastructure of the 

 cilia suggest that tlie molecular organization 

 of both cilia and myofibrillae of the muscle cells 

 are homologous and that the mechanism of their 

 contraction is similar. This conclusion gains 

 further support from bioclieniical studies which 



134 



FISH AND WILDLIFE SERVICE 



