MOLLUSC A. 39 



contrast between the two animals lies also in the dif- 

 ferences which may be observed in the marks left on 

 the shell by the muscles of the various parts, particu- 

 larly of the funnel. 



After these general comparisons, the mantle can be 

 stripped off from one side, exposing the parts within. 

 Here similar parts to those of the oyster will be ob- 

 served (Fig. 4). 



The gills, the palpi, the mouth, and the body are 

 similarly situated, but the heart has taken a position 

 such that the intestine passes through the ventricle, 

 instead of back of it. 



The gills will be found less complicated than those 

 of the oyster, but much more difficult to understand 

 than those of Pecten. They are free near the mantle 

 for a considerable space, and also around the foot, but 

 united posteriorly to the mantle and to each other in 

 the centre like those of the oyster. In fact, Pecten is 

 by far the easiest type for the study of all characteris- 

 tics, and if it had been as plentiful as the oyster, I 

 should have been tempted to take it as the type. 



If the muscles of the funnel (Fig. 3, rs) are lifted, 

 and probes or bristles passed through (as in Fig. 4, rj-'), 

 it will be found that it is really two tubes soldered to- 

 gether, one leading into the gill chamber, and the other 

 into the gill cavity. 



Then, if a live clam be observed, and blueing dropped 

 into the water near the openings of the siphon, the 

 grains will enter the gill-chamber tube, and presently 

 some of them be thrown out of the other. This shows 

 at once that they passed through the gills, and that 

 these siphons are merely prolongations of the mantle, 



