MOLLUSKS 119 



" plate gills," and the gills of a clam or oyster bear a close super- 

 ficial resemblance to a pair of delicate leaves on each side of the 

 body. In some of the lamellibranchs, however, the gills consist of 

 delicate filaments calling to mind the barbules of a feather. Gills 

 of this sort are seen in the jingle shell (Anomia), the bloody clam 

 and in mussels. In oysters and clams, however, the feathery fila- 

 ments are fused one to another at many points, leaving sieve-like 

 openings through which water may pass in its course through the 

 gills. The gill sheets are also fused to the mantle, thus forming a 

 chamber through which the waste water from the gills passes 

 along the sides of the Ijody on its way to the excurren-t tube of the 

 siphon. The water current over the gills is maintained by the 

 orderly and constant beating of vast numbers of cilia that cover 

 their surfaces. 



Lamellibranchs feed upon minute organisms, both animal and 

 vegetable. This food is drawn in with the water currents, and is 

 caught upon the slime of the gills, and then driven toward the 

 mouth by the movements of the cilia. There it is collected upon 

 the curtain-like lips, and driven by their cilia into the alimen- 

 tary tract. 



There is a fairly well developed stomach which is surrounded 

 by a large liver, the duct of which empties into the stomach. A 

 long gelatinous rod called the crystalline stylet is often found in 

 the stomach, but its exact nature and function are unknown. The 

 intestine is twisted, and is surrounded by the genital organs. 



The heart is three-chambered, and the hind-gut passes through 

 it. It pumps blood from the gills to other parts of the body. 



The LcDnellihrancliiata are sedentary creatures and many of 

 them remain fixed from the end of their larval development until 

 death, as is the case with the oyster. A few, such as the scallop, are 

 enabled to swim in an awkward darting manner by the rapid 

 closure of their valves, thus driving the animal hinge-side forward. 

 Others, such as the clams, are enabled to burrow through sand and 

 mud by means of their muscular foot, while still others, such as the 

 mussels, drag themselves slowly about by means of their byssus 

 threads. 



As one would expect in such slow moving creatures, sense or- 

 gans are poorly developed. On the mantle edge of the scallop, 



