Burrowing and Stationayij Mollusca, 353 



commend to your choice, for neither of them exactly cor- 

 respond with the observations I have made to satisfy myself; 

 but, my examination having been confined to the common 

 muscle (Mytilus edulis), I will not aver that the following 

 description of this part is generally applicable. From each 

 side of the shell, in front of the great adductors, a cylindrical 

 tendinous muscle {Jig, 60. a) arises, and, running forward 

 ^ obliquely, it meets its fellow 



tnear the centre, and opposite 

 to the hinge, where they 

 unite, and where they are 

 met by other two similar 

 muscles {b\ which arise near 

 the beaks, anterior to the 

 c? lesser adductors, and run 

 backward. From the place 

 of union between these mus- 

 cles originates the byssus {c) 

 by a single root or stalk. 

 This is firm, cylindrical, 

 cartilaginous, and of a clear 

 amber colour, continuing 

 simple for a short space, 

 when it divides, in a very irregular manner, into a few 

 branches, which are again divided into numerous entangled 

 threads. These are attached to the foreign external bodies 

 by means of the foot, a tongue-shaped organ lying at the base 

 of the byssus, distinguished by its dark violet colour, and 

 capable of considerable extension and retraction {d). There 

 is a furrow drawn along its middle, probably of use in holding 

 the threads while they are fixed without ; but I do not perceive 

 any glandular apparatus by which the latter might be secreted, 

 unless this should be a fleshy sheath, which, indeed, does sur- 

 round the base ; and the transition, from the peculiar structure 

 of the muscle to the horny structure of the byssus, is so 

 abrupt, that this looks rather like a new organ than a modi- 

 fication of the one to which it is attached, as Blainville sup- 

 poses. The accompanying figure will give you a good idea 

 of what I have just described. These byssiferous species 

 cannot voluntarily detach themselves; but if forcibly torn 

 away from their hold they can refix the shell, probably by 

 forming a new byssus ; a provision without which they must 

 have become the sport of the waves. 



The next tribe to which I direct your attention are of sin- 

 gular habits. They are not mechanically fixed, but they 

 Vol. IV. — No. 20. a a 



