Correct Option: B. Snail Detailed Explanation: Understanding Schistosoma: Schistosoma, commonly known as Bilharzia, is a genus of parasitic worms that cause schistosomiasis in humans. The life cycle of Schistosoma is complex and involves multiple stages, including both definitive and intermediate hosts. Life Cycle of Schistosoma: 1. Definitive Host: The adult Schistosoma worms live in the blood vessels of the definitive host, which is typically a human. Here, they reproduce and release eggs. 2. Eggs: The eggs are excreted from the host through urine or feces, depending on the species of Schistosoma. 3. Hatching: When the eggs come into contact with freshwater, they hatch into free-swimming larvae called miracidia. 4. Intermediate Host: The miracidia must find and penetrate a suitable intermediate host, which is usually a specific species of freshwater snail. Inside the snail, the larvae undergo several developmental stages, eventually producing another larval form called cercariae. 5. Cercariae: The cercariae are released from the snail into the water, where they can infect humans by penetrating the skin when a person comes into contact with contaminated water. Why Option B (Snail) is Correct: - The snail is a crucial intermediate host in the life cycle of Schistosoma. It is where the larvae develop and multiply before transitioning to the next stage that infects humans. Without the snail, the life cycle of Schistosoma cannot continue, making it an essential component of their biology. Why Other Options are Incorrect:

• Option A: Man
• While humans are the definitive hosts for Schistosoma, they are not intermediate hosts. In the life cycle, humans are where the adult worms live and reproduce, but they do not serve as a stage for larval development.
• Option C: Mosquito Larva
• Mosquito larvae are not involved in the life cycle of Schistosoma. They do not serve as hosts for any stage of the Schistosoma life cycle. Mosquitoes are vectors for other diseases (like malaria), but they have no role in schistosomiasis.
• Option D: Crayfish
• Crayfish are not part of the Schistosoma life cycle either. They do not serve as hosts for the larvae and are not involved in the transmission of the parasite. Like mosquitoes, they are unrelated to the biology of Schistosoma.

Summary:

• The correct answer is B. Snail, as it is the intermediate host in the life cycle of Schistosoma.
• Schistosoma requires a snail to develop from larvae to cercariae before infecting humans.
• Humans are the definitive hosts, not intermediate hosts.
• Other options (man, mosquito larva, crayfish) do not play a role in the life cycle of Schistosoma.

Revision Summary:

• Schistosoma (Bilharzia) has a complex life cycle involving both definitive and intermediate hosts.
• The snail is the intermediate host where larval development occurs.
• Humans are the definitive hosts where adult worms live and reproduce.
• Other options (man, mosquito larva, crayfish) do not serve as intermediate hosts for Schistosoma.