Question

The stage absent in the life cycle of Schistosoma is
(a) Miracidium
(b) Redia
(c) Cercaria
(d) Both B and C

Answer 1

Hint: Schistosoma also called blood fluke is an endoparasite in humans. It causes schistosomiasis in which the intestine or the urinary tract is infected. It is a digenetic parasite and requires more than one host to complete its life cycle.

Complete answer:
When the eggs of this endoparasite are released into the water through feces, the mature miracidium hatches out of the egg. This hatching of the egg depends upon the temperature, light, and dilution of feces in the water. Now, the miracidium searches for a freshwater snail belonging to the genus Biomphalaria. The miracidium penetrates into the soft tissue of the snail. After it enters the snail’s body, it loses its cilia and develops into mother sporocysts. The sporocysts rapidly multiply through asexual reproduction which results in numerous daughter sporocysts. These daughter sporocysts move to the liver and gonads of the snail for further growth. After 2-4 weeks, they undergo a metamorphosis which gives rise to a fork-tailed cercaria. These cercaria penetrate out of the snail into the water after they get stimulated by light.

Additional information: Redia is also a trematode larval stage. After the sporocysts from the larva, the first development from it forms the redia. It gives rise to other redia or to a different larva(cercaria).
So, the correct answer is ‘(b) Redia’.

Note: Miracidium, Rediae and Cercaria are larval stages in the life cycle of trematodes. Schistosoma is also a genus of trematodes but unlike other members, it has only two larval stages - Miracidium and Cercaria.