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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 certain seasons of the year, usually in late summer, these snail hosts shed thousands 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

Figure 1. 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 proboscis and stabbed into the skin of the victim.