Microns 



Figure 126. — Transverse section of the principal filament of the gill of C. virginica drawn from the same preparation as 

 in figure 125. Note the well-developed muscle fibers, m., under the large skeleton bars, ch.r.. c.t. — connective 

 tissue; bl.v. — blood vessel; bl.s. — blood space; fr.c. — frontal cilia; If.c. — laterofrontal cilia; I.e. — lateral cilia; m. — 

 muscle. 



groove. This depression at the border of the 

 gills extends their entire length. The epithelial 

 lining of the terminal groove consists of columnar 

 ciliated cells with large cilia and numerous mucous 

 and eosinopliihc cells. The epithelium rests on 

 a basal membrane. Transverse muscle fibers 

 extend between the two sides of the groove. 

 During feeding the grooves are open, the condition 

 which is shown in figure 128. Their contraction 

 brings the edges together and closes the groove. 

 In this way the oyster discards some of the 

 material which was collected by the surface of 

 the gill. The rejected particles entangled in 

 mucus are dropped to the inner surface of the 

 mantle and are discharged. The direction of the 

 ciliary beat along the four terminal grooves is 

 always toward the labial palps and the moutli. 



THE MUSCLES OF THE GILLS 



The gills of an actively feeding oyster contract 

 and expand at frequent, although irregular, inter- 

 vals. This behavior is difficult to notice in an 



intact oyster, but it can be observed in an oyste^" 

 in which much of the valve has been cut off 

 without injuring the adductor muscle and the 

 gills. The mantle at the exposed area rolls up 

 and leaves tiie gills in full view, and if carefully 

 performed, the operation has no visible ill effect 

 on the function of tlie gill. 



The most conspicous movements which can be 

 seen with the naked eye are the muscular con- 

 tractions at the bases of the gills and the corres- 

 ponding changes in the position occupied by the 

 demibranchs. These four structures may stand 

 apart like stiff leaves of a wide open album or 

 tiiey remain parallel, touching one another like 

 the pages of a closed book. There is also a lateral 

 movement of the filaments which brings them 

 together or pushes them apart. This movement 

 frequently occurs independently of the contrac- 

 tions of tlie demibranchs and nnvy be limited to 

 a small portion of the plica. Both types of move- 

 ments afl'ect the opening of the ostia, which are 

 widely stretched when either the four demi- 



130 



FISH AND WILDLIFE SERVICE 



