MOLLUSCS. 



M 



undergoes a considerable metamorphosis, replacing almost every feature of 

 its anatomy with new structures, until at last it drops from the fish to the 

 bottom, and there slowly grows into the adult. The parent mussel is also 

 capable of recognizing the presence of fishes ; for if none are present, the 

 embryos are retained for a long time in the branchial pouches. 



The salt-water mussels are more like the forms already enumerated 

 than are their fresh-water namesakes. They occur in all the northern 

 seas south to about the thirty-fifth parallel of latitude. In some places 

 they form enormous beds on the mud-flats, or attached to rocks and piles, 

 seeming to prefer shallow estuaries, where there is considerable mud. On 

 examining one of these mussel-beds, it is quickly seen that each individual 

 shell is firmly anchored to some other object by means of a bundle of 



Fig. 82. — Common salt-water mussel (Mytilvs edulis) attached to a branch 

 by means of its byssal threads. 



silken cords. This bundle is the ' byssus,' to which reference has already 

 been made. These silken threads are the product of a gland in the foot. 

 When first secreted, they are fluid in nature, but contact with the water 

 immediately hardens them. Besides serving to moor the shells, these 

 byssal threads can even be used as organs of climbing. The animal 

 extends its foot upwards as far as possible, presses it against some pile or 

 rock, and then draws out one of these silken threads ; again and again the 

 operation is repeated, the elasticity of the united threads serving to lift 

 the animal a little and allow him each time to extend his foot to a higher 

 point on the pile than before. Gradually in this way the shell is lifted, 



