The smaller female parasite lives inside the thicker male
Scientists have decoded the genetic blueprint of two parasitic flatworms responsible for thousands of deaths worldwide every year.
Schistosoma mansoni and Schistosoma japonicum both cause the debilitating disease schistosomiasis, otherwise known as Bilharzia.
The work has already uncovered targets for new treatments for the disease, which causes fever and fatigue.
The international study features in the journal Nature.
Schistosomiasis cases top 200 million every year, with 20 million people are seriously disabled by severe anaemia, chronic diarrhoea, internal bleeding and organ damage caused by the worms and their eggs, or the immune system reactions they provoke.
In sub-Saharan Africa alone it kills 280,000 people each year.
People become infected with Schistosoma when they wade or bathe in water inhabited by tiny snails that host the parasites
The parasites are released into the water, and use fork tails to burrow into the skin
They travel to blood vessels that supply urinary and intestinal organs, including the liver, where they mature
Female worms, which live inside the thicker males, release many thousands of eggs each day
Eggs shed in urine and faeces may make their way into snail-inhabited water, where they hatch to release parasites that seek out snails to begin the cycle again
Dr Anthony Fauci, director of the US National Institute of Allergy and Infectious Diseases, said: "Chronic infection with Schistosoma parasites makes life miserable for millions of people in tropical countries around the globe, and can lead to death.
"New drugs and other interventions are badly needed to reduce the impact of a disease that lowers quality of life and slows economic development."
Since the 1980s, a cheap drug, praziquantel has been widely distributed to areas where the disease is common.
However, although the drug is effective, it does not prevent a person becoming re-infected. There is also a risk that the parasites will become resistant to it.
Therefore, developing new drug targets is important.
Researchers working on the genetic blueprint of S. masoni, the most widespread of the schistosomiasis parasites, found that it was made up of 11,809 genes - about 10 times the size of the malaria parasite genome.
In particular, they identified a large number of genes which produce enzymes that break down proteins, giving the parasite its ability bore through tissue.
Subsequent analysis revealed 120 enzymes that could potentially be targeted with drugs to disrupt the worm's metabolism.
The researchers also identified 66 drugs already on the market which might also be effective against schistosomiasis.
The analysis also found that S. mansoni lacks a key enzyme needed to make essential fats, and must rely on its host to provide these - revealing a potential Achilles' heel that could be exploited for drug development.
Researcher Dr Matthew Berriman, of the Wellcome Trust Sanger Institute, said: "This genome sequence catapults schistosomiasis research into a new era.
"It provides a foundation for understanding aspects of the parasite's complex biology as well as a vehicle to immediately identify new targets for drug treatment."
Fellow researcher Dr Najib El-Sayed, of the University of Maryland, said: "The genome sequence has given us, for the first time, a comprehensive view of the engines that drive the parasite, the strategies that allow it to survive in us, its human host.
"It is a catalogue of opportunities."
In a separate study, scientists discovered that S. japonicum, which is largely confined to Asia, had even more genes.