64 



FIRST LESSONS IN ZOOLOGY. 



Here it will lie, with its short siphon extended at the level 

 of the sand, as in the sketch. 



The shell is lined with a thick nacre, or pearl, and often 

 good salable pearls will be found inside the shell. 



The common edible sea-mussel (Mytilus, Fig. C4) should 

 also be examined, as it can be easily obtained attached to 

 wharves, or at low water, where it grows in such great 



FIG. 65. Common mussel, , mantle; 6, foot; c, byssus; d and e, muscles re- 

 tracting the foot; /, mouth; 17, palpi; h, visceral mass inclosing the stomach 

 and liver; i, inner gill; j, outer gill. 



quantities as to form " mussel-banks," attached to one an- 

 other or to stones, etc., by threads called the "byssus." 



The mussel's shell is rather a simple affair. It has a 

 very slight hinge, with no teeth, while the beaks are large, 

 but not distinct from the rest of the shell. 



The shell is easily opened by cutting the muscles at the 

 anterior or smaller end of the shell, and the different parts 



