VENUS MERCENARIA II 



This groove is also lined by ciliated cells, and the whole mass is 

 swept swiftly forward in it toward the palps. The natural food 

 of the clam, of course, is carried forward in the same way. It is 

 evident that a large proportion of the organisms floating in the 

 water which enters the mantle chamber must come in contact with 

 the sides of the gills, and be carried forward to the mouth folds, 

 to which they may be transferred. 



These points may be made more clear by referring to the diagram 

 [fig. 4] . It represents a section made transversely across the fila- 

 ments of a typical lamellibranch gill. In a single gill there are 

 thousands of these rods. But five are shown here on each side, 

 standing in row to form the perforated walls of the gill. Each rod 

 is represented as being more or less oval, when its cut end is 

 viewed in this way. In three places are shown the lateral union 

 of filaments. The reference letters ig are supposed to be placed 

 in the interior space of the gill, and p shows the nature of the 

 partition, or septum, which, at more or less regular intervals, 

 stretches across this space and holds the two walls of the gill 

 together. 



The details of cellular structure have been drawn in two 

 filaments. The long, straining cilia, which stretch across the spaces 

 between rods, are shown at sc, and the arrow indicates the course 

 taken by the water current as it enters the interior of the gill. The 

 cilia which cause this entering current are the frontal cilia, fc. 

 Opening on the surface between them and the straining cilia are 

 the gland cells, gc, the secretion from which cements together the 

 food particles. 



This figure is not intended to represent the details of structure 

 found in the gill of Venus, which is much more complicated in 

 q^inv ways. The general plan of structure and of function in that 

 form, however, is very much as represented, and this diagram is 

 used because it may be so much more easily described. 



If we now examine the palps with a hand lens, we may notice 

 that their inner surfaces — those nearest to the mouth — are 

 covered by a set of very fine parallel ridges. The lateral portions 

 of the palps are shown in figure 2, ap and pp. They are capable of 

 many movements. They may be bent and spirally twisted, 



