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 
Mytilus edulis, 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 Mytilide 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 Mytilide 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 Mytilus 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. 
