The sea mussel (Mylilus edulis) with foot extended in the act of attaching a byssus thread. At the base of 

 the foot may be seen the byssus organ projecting between the valves and also the byssus which anchors the animal 

 firmly to the rocks by scores of tenacious fibers 



The ability to creep about thus, readily 

 enables the mussel to migrate from 

 unfavorable surroundings and locate 

 in the most advantageous situations. 



When the ideal position is found, the 

 mussel secretes a number of tough elas- 

 tic threads forming the byssus, which 

 firmly anchors it in place.^ A single 

 thread may be formed and attached in 

 three minutes but usually more than 

 double that time is taken. As many as 



'The byssus is composed of three parts; a stalk, a 

 thread, and an end plate which securely cements the 

 thread to some solid object. Each of these is secreted 

 by a separate gland in the foot. The stalk comes from 

 a large gland situated in the base of the foot. Micro- 

 scopic sections show that it is composed of many 

 glandular membranes that hang down in a byssus 

 cavity like a series of curtains. They secrete a sticky 

 fluid that fills the cavity and is forced out at the base 

 through a round aperture which molds it into the form 

 of a rod. Contact with sea water causes this sticky 

 fluid to become as firm and tough as horn. 



The secretion that forms the byssus thread arises from 

 glands in the foot which discharge into a ventral groove 



eighteen threads have been formed by a 

 mussel under observation during a period 

 of four hours, and as many as one hundred 

 of these threads may be found anchor- 

 ing the mollusk firmly to its neighbors 

 or to other points of attachment. In a 

 mussel community all the members in 

 contact with one another are firmly 

 woven together by means of the byssal 

 threads. Under this condition power- 

 ful waves, swift currents and even ice 

 have little effect on the firmly united 

 community. 



"When we speak of the gills of an animal 

 we usually think of them as respiratory 

 organs but in the case of the mussel the 



running the entire length of the foot where it is molded 

 into shape and hardened by contact with the sea water. 

 The attachment plate is formed from secretions pro- 

 duced in the upper part and end of the foot by glands 

 that discharge into the hollow depression near the tip 

 on the under side. 



361 



