<P> All octopuses are venomous, but only a few species pose a significant threat to humans . Blue - ringed octopuses in the genus Hapalochlaena, which live around Australia and New Guinea, bite humans only if severely provoked, but their venom kills 25% of human victims . Another tropical species, Octopus apollyon, causes severe inflammation that can last for over a month even if treated correctly, and the bite of Octopus rubescens can cause necrosis that lasts longer than one month if untreated, and headaches and weakness persisting for up to a week even if treated . </P> <P> All species of cone snails are venomous and can sting painfully when handled, although many species are too small to pose much of a risk to humans, and only a few fatalities have been reliably reported . Their venom is a complex mixture of toxins, some fast - acting and others slower but deadlier . The effects of individual cone - shell toxins on victims' nervous systems are so precise as to be useful tools for research in neurology, and the small size of their molecules makes it easy to synthesize them . </P> <P> Schistosomiasis (also known as bilharzia, bilharziosis or snail fever), a disease caused by the fluke worm Schistosoma, is "second only to malaria as the most devastating parasitic disease in tropical countries . An estimated 200 million people in 74 countries are infected with the disease--100 million in Africa alone ." The parasite has 13 known species, two of which infect humans . The parasite itself is not a mollusc, but all the species have freshwater snails as intermediate hosts . </P> <P> Some species of molluscs, particularly certain snails and slugs, can be serious crop pests, and when introduced into new environments, can unbalance local ecosystems . One such pest, the giant African snail Achatina fulica, has been introduced to many parts of Asia, as well as to many islands in the Indian Ocean and Pacific Ocean . In the 1990s, this species reached the West Indies . Attempts to control it by introducing the predatory snail Euglandina rosea proved disastrous, as the predator ignored Achatina fulica and went on to extirpate several native snail species, instead . </P>

What are the classes of the phylum mollusca