Marvels of the Universe 



269 



Pholo bo'] [11. ,Saiille Kent. 



A GIANT CLAM. 



In this photograph the Giant Clam is alive, and between the partly open valves of the shell the "mantle" which line 

 and produces the shell is seen. The weight of the shell keeps it in position among the rocUp. 



THE GIANT CLAM 



BY SIR H. H. JOHXSTOX, G.C.M.G. 



The Giant Clam, or Tridaciia, like the familiar oyster, is a bivalve, and is the supreme develop- 

 ment in size known amongst all shells. The specimens illustrated on page 270 measure 

 about three feet across at their widest ; but according to the late W. Saville Kent, probably 

 the largest may attain to ten feet in width off the coast of Queensland, and the entire animal 

 weighs as much as two thousand pounds. These Clams are evidently among the latest 

 developments of bivalve shells, since they cannot be traced back farther geologically 

 than the Miocene Period or middle of the Tertiary Epoch, The muscular power developed 

 in these huge Clams is very great. When undisturbed this mighty Clam usually remains with 

 the shell partially open, but if. a man were so foolish as to insert his hand or foot between the inter- 

 locking valves they would at once close on such an intrusion, and it would be impossible to open 

 them without recourse to the forcible application of crowbars or axes. The Clams of to-day — or, 

 at least, those of enormous size — come mainly from the shallow parts of the Pacific Ocean, amongst 

 the coral islands ; and Mr. Saville Kent, in his monumental work, " The Great Barrier Reef of 

 Australia," alludes to the loss of life occasioned among the men who wade or dive among the coral 

 reefs in search of pearls or beche-de-mer , by their inadvertently inserting a foot between the gaping 

 valves of the Giant Clam, The outer surface of the shell in the older clams is so disguised by a 

 parasitic growth of weeds, coral and sponges that they are not easily distinguishable from their 

 inanimate surroundings, and the " mantle " of the Giant Clam, which may be seen between the 

 open valves, is not brilliantly coloured like that of the smaller species, but is only light brown, with 

 stripes of darker brown. The fisherman — black, white or yellow — once caught by the Clam, is held 



