Enemies of the American Oyster 163 



masses of vegetation. Sometimes oyster shells become 

 covered by such weeds as the sea lettuce (Ulva), spread- 

 ing out great sheets of tissue that are held flat on the 

 bottom in tide currents. 



These plants are not parasitic, but may interfere with 

 oyster growth by preventing the access of food-bearing 

 currents. When oysters so covered are dredged, it is 

 necessary to strip the weed off, and in the Gulf, certain 

 weeds with cylindrical fibers are brittle enough to break 

 into small pieces, and these, getting into the " meats " 

 during the shucking process, though harmless, make 

 them quite unsalable. From this cause beds are some- 

 times temporarily abandoned. 



In the brackish waters in which oysters thrive best, the 

 shells rarely become covered with a growth of hydroids. 

 These animals with their root-like bases, their branching 

 bodies, and beautifully colored spreading zooids, greatly 

 resemble gardens of miniature flowering plants; but 

 they have no beauties for the oysterman. Feeding on 

 minute swimming animals, and merely resting on the 

 shells, they do not harm the oysters, but it is difficult to 

 clean the shells of them, and both on " shell stock " and 

 oysters for shucking they are a nuisance. 



Like other animals, oysters seem to be subject to cer- 

 tain diseases, one of which, at any rate, is known. In 

 the Chesapeake, and probably all waters to the south, 

 there sometimes appears in the pulpy visceral mass, a 

 swarm of small worm parasites. These seem not to be 

 known to oystermen, who may, however, have noticed 

 the very poor condition of certain individuals that are 

 really diseased. It was left to a biologist to show that 

 disease existed. 



The presence of the parasite Bucephalus prevents the 



