VENUS MERCENARIA 9 



of extremely fine rods placed side by side, as represented 

 in the most diagrammatic way possible in figure 3. In order that 

 these rods, r, may retain their position, they are in many forms, 

 irregularly fused with each other by secondary lateral growths of 

 tissue, ic. The outer and inner walls of the gill are also held 

 together by partitions which extend across the inner space between 

 them, p. The gill is thus seen to be basketHke, the walls being 

 made of rods between which are spaces, s, which put the interior 

 chamber in communication with the mantle space in which the 

 gills hang. 



These rods, or filaments, of which the gill is made, contain an 

 interior space in which the blood flows. They were probably 

 primarily developed in order that the blood of the body might be 

 brought in close contact with the water, that, by diffusion, the 

 carbon dioxid of the blood might pass outward through the thin 

 walls, while, by the same process, oxygen, carried by the water, 

 might pass into the blood. But, in addition to performing the 

 function of breathing, the gills have taken on that of collecting 

 minute organisms used as food. This is accomplished by a com- 

 plicated process. 



We have seen that a constant stream of water entered the mantle 

 or branchial chamber. What becomes of it? And what is it that 

 causes the current? All of this water in the mantle chamber streams 

 through the minute openings between the filaments of the gill and 

 enters its interior space. It now rises to the base of the gill, and 

 flows into a tube, the epibranchial chamber [fig. i, ec], through 

 which it passes backward, leaving the body by the upper or exhalent 

 siphon, which is directly continuous with the epibranchial chambers 

 of the four gills. The currents which we first noticed, then, enter 

 the mantle chamber by the lower siphon, pass into the -interiors of 

 the four gills, flow to their upper or attached edges, and are directed 

 backward and out through the upper siphon tubes. of the mantle. 



The cause of these rapid currents is revealed by a microscopic 

 examination of the rods or filaments of the gills. These are found 

 to be covered on their outer surfaces, which face the water on 

 both sides of the gill, with innumerable short, hairlike structures 

 which project perpendicularly from the surface. These cilia 



