Question

Sequentially arrange the following stages in order of their appearance in the life-cycle of Fasciola gigantica:
(A) Miracidium
(B) Redia
(C) Cercaria
(D) Sporocyst
(E) Metacercaria
Choose the correct answer from the options given below:

• A. (A), (B), (C), (D), (E).
• B. (A), (D), (B), (C), (E).
• C. (B), (A), (D), (C), (E).
• D. (A), (B), (D), (E), (C).

Hint:

To remember the trematode larval stages in order, use the classic mnemonic: "Many Spicy Red Cardinals Melt" $\rightarrow$ Miracidium, Sporocyst, Redia, Cercaria, Metacercaria.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
Fasciola gigantica is a giant liver fluke of ruminants that has an indirect life cycle requiring a freshwater snail (typically Lymnaea auricularia) as its intermediate host.
The parasite undergoes several distinct developmental stages to complete its life cycle.

Step 2: Detailed Explanation:
Let us trace the chronological sequence of developmental stages in the life cycle of Fasciola gigantica:
1. Miracidium (A): Eggs are passed in the feces of the host and hatch in water, releasing a free-swimming ciliated larva called a miracidium. This larva locates and penetrates the tissues of the intermediate snail host.
2. Sporocyst (D): Once inside the snail, the miracidium loses its cilia and transforms into a sac-like stage called a sporocyst, which contains germinal cells.
3. Redia (B): The germinal cells inside the sporocyst develop into the next larval stage, the redia, which possesses a mouth and gut to feed on snail tissues.
4. Cercaria (C): The rediae produce free-swimming, tailed larvae called cercariae, which escape from the snail into the water.
5. Metacercaria (E): The cercariae swim to aquatic vegetation, lose their tails, and encyst to form infective metacercariae, which are then ingested by grazing ruminants.
Therefore, the correct developmental order of these stages is Miracidium (A) $\rightarrow$ Sporocyst (D) $\rightarrow$ Redia (B) $\rightarrow$ Cercaria (C) $\rightarrow$ Metacercaria (E).

Step 3: Final Answer:
The correct sequential arrangement is (A), (D), (B), (C), (E), which corresponds to option (B).