Figure 137. — Diagram of the system of ciliary currents 

 on the surface of the demibranch of C. virginica. The 

 four plicae are shown slightly pulled apart to indicate 

 the principal (wide) filaments at the bottom of the 

 grooves. Open ostia, o., are shown only on the left 

 plicae; the mouth is toward the left; b. — base of the 

 gills; or.f. — ordinary filaments; o. — ostia; pr.f. — prin- 

 cipal filament; tr.g. — terminal groove. 



by the frontal cilia of the principal filaments 

 (pr.f.) runs in the opposite direction, i.e., from 

 the free edge of the gill toward the base. Particles 

 carried by this current enter the track along the 

 base of the gills (b.), which rmis parallel to the 

 du-ection of the current in the terminal groove and 

 carries food particles toward the mouth. The 

 lateral cilia (not shown in the diagram) beat at 

 right angles to the surface of the gill and create a 

 cm'rent that forces water inside the water tubes 

 and into the epibranchial chamber. 



Small single particles fall into the grooves and 

 eventually are carried by the principal filaments 

 toward tlie mouth while the larger particles or a 

 mass of small ones entangled in mucus are pushed 

 by the frontal cilia toward the free edge of the 

 demibranch and may be dropped from the gill 

 before entering the terminal groove. Frequently 

 a group of particles is passed from the edge of one 

 demibranch to the surface of the underlying one 

 before it is discarded. The complex system of 

 ciliary currents in the gill constitutes an efficient 

 selective mechanism for the sorting of food. 

 Final selection is made along the surface of the 



labial palps, which reject a large portion of the 

 material brought in by the gills (see p. 115) 



The ciliary tracts of the gills of 0. edulis de- 

 scribed by Atkins (1937), in general resemble 

 those observed on the gill of C. m.rginica (fig. 138). 

 In the three species of oysters C. virginica, 0. 

 edidis, and C. angulata, the ciliation is essentially 

 the same. 



MECHANICAL WORK OF THE LATERAL 

 CILIA 



The lateral cilia function principally as movers 

 of water. They force water tlu-ough the ostia into 

 the water tubes of the gills and maintain inside 

 the gill a current that passes through the branchial 

 chambers to the outside. The hydrostatic pres- 



l-f.C. 



pv-o-l-f.e. 



i.f. 



etc. 



^^;^ 















■;i^<: ^\f/< 



1 



20 



Microns 



Figure 138. — Frontal view of a living filament of O. edulis. 

 c.f.c. — coarse frontal cilia; f.f.c. — fine frontal cilia; If.c. — 

 laterofrontal cilia; pro. If.c. — subsidiary laterofrontal 

 cilia. From Atkins, 1937, figure 1. 



THE GILLS 



143 



