MOLLUSCS. 91 



yet settled beyond a doubt. Various explanations have been advanced, 

 but none are perfectly satisfactory. Some believe that the edges of the 

 valves are the effective agents ; but were this the case, one would expect 

 them to be themselves abraded far more than they really are. Some sup- 

 pose that the mollusc secretes an acid which softens or dissolves the rock ; 

 but no one has yet proved the presence of such an acid in the secretions of 

 these burrowing animals, although they occur in a few other forms, which 

 never form burrows. A third, and the most satisfactory, explanation is 

 that the foot is the burrowing apparatus, and this receives some confirma- 

 tion from the fact that in these forms it is covered with a hard cuticle. 

 The operation may be accelerated by grains of sand getting between the 

 foot and the rock, and by constant motion wearing away the substance. 

 There is another feature connected with these burrowing molluscs, for 

 which it is not easy to assign a reason, — all of them are highly phos- 

 phorescent. 



Dreysseus was a celebrated Belgian physician, and his name modified 

 into Dreissensia is given to a mollusc which we must not neglect. In 

 shape it is much like a common marine mussel, and like it, it forms a 

 byssus. It is, however, a fresh-water form. It was first discovered, in the 

 Aral and Caspian seas, by Pallas, a naturalist who traveled in the East 

 near the close of the last century. From the Caspian it gradually ex- 

 tended its range up the Volga and its tributaries, and in some way or other 

 gained entrance to the European river systems, and to-clay it is distributed 

 over almost the whole of Europe. It is a most wonderfully variable form, 

 and in the early clays each naturalist who found it imagined he had some- 

 thing new, and hence it has been described under a host of names. 



The largest of all living bivalves is the giant clam of the South Seas. 

 Alexander the Great at one time sent a naval expedition to Southern India, 

 which brought back many wonderful things, as well as tales, of what they 

 saw. Among the other stories was one of an oyster so large that it re- 

 quired three bites (tri dacno) to eat it. Alexander and all his hosts passed 

 away, but the story lived ; and so when the explorers brought home speci- 

 mens of the giant clam from the far East, there was no alternative, — 

 they had to be called Tridacna. Still, the term is not strictly accurate, 

 for it would require considerably more than three ordinary mouthfuls to 

 to make way with a clam, the fleshy parts of which occasionally weigh 

 twenty pounds. The shells are proportionately large, and occasionally a 

 pair of valves will be found which will weigh five hundred pounds. Besides 

 the flesh (which is said to be good) the shell of the giant clam is of value 

 to the natives of the coral islands of the Pacific. Stone is entirely lacking 

 with them, but this shell takes its place. From it they make their knives, 



