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 a definitive host and an intermediate host. 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. Intermediate Host: The eggs hatch in water, releasing larvae called miracidia. These larvae must find and infect a suitable intermediate host, which is a specific type of freshwater snail (e.g., Biomphalaria species). 4. Development in Snail: Inside the snail, the miracidia develop into another larval stage called cercariae. This process can take several weeks. 5. Cercariae Release: The cercariae are then released back into the water, where they can swim and penetrate the skin of a human host, continuing the cycle. Why Option B (Snail) is Correct: - The snail is the only intermediate host in the life cycle of Schistosoma. It is essential for the development of the parasite from the larval stage to the stage that can infect humans. Without the snail, the life cycle cannot continue, and the parasite cannot propagate. Why Other Options are Incorrect:

• Option A: Man
• While humans are the definitive hosts for Schistosoma, they are not intermediate hosts. The role of the definitive host is to provide a suitable environment for the adult worms to live and reproduce. Therefore, this option is incorrect.
• Option C: Mosquito Larva
• Mosquito larvae are not involved in the life cycle of Schistosoma. They do not serve as hosts for the parasite at any stage. This option is incorrect because it does not relate to the biology of Schistosoma.
• Option D: Crayfish
• Crayfish are not part of the Schistosoma life cycle. They do not serve as intermediate hosts for the parasite. This option is incorrect as well, as crayfish do not play any role in the transmission or development of Schistosoma.

Summary:

• The intermediate host for Schistosoma is the snail, specifically certain freshwater species.
• Humans are the definitive hosts where adult worms live and reproduce.
• Other options (man, mosquito larva, crayfish) do not play a role in the life cycle of Schistosoma.
• Understanding the life cycle of Schistosoma is crucial for grasping how schistosomiasis is transmitted and how to control it.

Revision Summary:

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