PELECYPODS 427 



ceedingly tough, fibrous byssus. By means of this the animal may 

 not only attach itself firmly to' any sort of object, but may actually 

 move about. Fibers of the byssus are thrown out, and, as though 

 possessed of life, they seize upon objects in the direction in which 



JUytilus edulin, typical form. 



Mytilus desires to move, and by contraction of the muscles which 

 control the byssus the animal is drawn forward, the operation 

 being then repeated. It must be a very tedious method of travel. 

 However, the Mytilidce are not restless creatures. They pass 

 their lives content to remain firmly fixed to some object, and 

 their movements, if any, are within a very narrow circle. The 

 animal has very long gills of the usual filamentous type. The 

 mantle fuses at one point to form an anal siphon, the lower or 

 branchial one being, therefore, functional. The palps are large. 

 One curious feature of this group is the enormous extent of the 

 genital glands, which actually appear to pervade the entire struc- 

 ture of the animal, gills, mantle, foot, and all. At times the 

 whole being is a veritable receptacle for myriads of tiny yellowish 

 eggs. 



The Mytilidce are of universal distribution, and are for the 

 most part of littoral or shallow- water range. Some species 

 are esteemed as an article of food, notably Mi/tiliis edulis. 



Mussel-culture has been profitably carried on in France for a 

 long time, but the genus has not been utilized to any extent as. 

 food in this country. 



