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4 American Seashells
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
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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.