Anatomy of the Food Mollusks 



35 



the filaments have grown together by their inner edges, 

 leaving slits here and there for the ingress of water. 



Figure 8 represents a cross 

 section of several filaments in the 

 fold of a lamella of the oyster 

 gill. Four of these filaments 

 have grown together along their 

 inner margins. If the section 

 had passed a little higher or lower 

 on the gill, one or more of these 

 would have been shown to be free 

 from the others. Several free 

 filaments are shown in the figure, 

 and between them water enters 

 the interior of the gill. But 

 above and below the plane of the 

 section, these also would be united 

 with contiguous filaments for 

 shorter or longer distances. 



Gill filaments, when greatly magnified (Figure 9), 

 show essentially the same structure in nearly all lamelli- 

 branchs. They are tubes for the circulation of blood, 

 and their walls are single layers of cells as shown in the 

 sectional views of the oyster and scallop gills. Each fila- 

 ment contains a pair of rods of secreted, rubbery sub- 

 stance that give stiffness to the slender tube, and probably 

 tend to keep its blood space (b) open. The cells of the 

 wall are modified on the outer edge of the filament. 

 Some of them bear an immense number of cilia (/ c), 

 which are protoplasmic hairs having an excessively rapid 

 lashing movement that produces currents in the water, 

 and also removes foreign particles from the surface of 

 the gill. At the margins of this tract are rows of cells 



Fig. 8. — Transverse sec- 

 tions of several fila- 

 ments of the oyster's 

 gill. A union of the in- 

 ner edges of four of 

 these is shown, and 

 their common blood 

 space is indicated at b. 



