4 American Seas hells 



poisonous micro-organisms. Prior to the advent of ships with metal hulls no 

 vessel on the seas was safe from the borings of molluscan "shipworms." 

 Many ships have disappeared at sea as a result of being weakened by the 

 attacks of these creatures. Even today damage to the extent of millions of 

 dollars is done every year to wharf pilings, small craft, and hemp lines by 

 these bivalves. In many parts of the world the health of millions is seriously 

 menaced by mollusks. It was not until the turn of the century when modern 

 research was directed toward tropical diseases that the full importance of 

 snails as carriers was appreciated. Six major parasitic diseases have been 

 shown to be transmitted by fresh-water mollusks. Thousands of people die 

 each year in China and Egypt from the blood-fluke disease alone. No fatal 

 snail-borne disease is present in North America proper, but visitors to the 

 West Indies and northern South America are warned to keep out of ponds 

 and flooded ditches in these regions. 



In other respects, mollusks are of minor medical importance. A number 

 of parasitic diseases of sea birds and fish are carried by marine shells, such as 

 the periwinkles Littorina and Tectarius and other shore species. During cer- 

 tain seasons of the year, usually in late summer, these snail hosts shed thou- 

 sands of microscopic larval worms into the sea water. Although normally 

 destined to penetrate the skin of birds, these tiny creatures sometimes attack 

 man and cause an uncomfortable rash or "swimmer's itch" which is often 

 mistaken for jellyfish sting. 



Among the most dangerous inhabitants of the coral reefs in the tropical 



SIPHON 



TeNTACLE 



/ 



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\PR060StlS . 



\ I 



RRDULHR 

 Tooth 



Figure i. The large cone shells of the Indo-Pacific, and possibly those of the 

 Atlantic, can inflict a serious, and at times fatal, sting. The venom leaves the poison 

 sac and, together with the tiny, harpoon-like tooth, is ejected from the snail's pro- 

 boscis and stabbed into the skin of the victim. 



