172 



GENERAL ZOOLOGY 



of the mussel, but as a matter of strict accuracy the mussel 

 does not have a siphon, in the sense that the clam possesses 

 one. The foot of the mussel is large and muscular. It 

 enables the animal to plow its way through mud or even 

 through heavy gravel. The gills and the palps are practically 

 identical in structure in the mussel, the clam, the oyster, and 

 the scallop. The internal organs also have the same general 

 plan of structure in the four animals named. There are two 

 adductor muscles in the mussel. The sexes are separate. 



FIG. 88. Living Fresh- Water Mussel. x| 



Development. At least two genera of fresh-water mussels 

 carry their young in their gills, which are open dorsally on 

 the inside. Such an arrangement is necessary because the 

 currents in rivers all go one way, and would carry the help- 

 less young out to sea. At a certain stage the well-developed 

 larvae escape from the brood-pouch of the female mussel 

 and fall to the bottom. Fish " nosing " along the river-bed 

 touch the floating byssus-thread of the young mussel, which 

 at that stage is called the glochidium (Fig. 89). The thread 



