Question

Which larval form of {Fasciola hepatica has a motile tail?}

• A. Miracidium
• B. Redia
• C. Sporocyst
• D. Cercaria

Hint:

Remember the motile stages in the fluke life cycle:

\textbf{Miracidium}: Motile by \textbf{cilia}.
\textbf{Cercaria}: Motile by a \textbf{tail}. \end{itemize} The stages inside the snail (Sporocyst, Redia) are non-motile sacs for asexual reproduction.

Answer 1

The Correct Option is D

Step 1: Understanding the Concept:
{Fasciola hepatica}, the common liver fluke, has a complex life cycle involving multiple larval stages and an intermediate host (typically a snail). The question asks to identify the specific larval stage that is characterized by the presence of a motile tail for locomotion.

Step 2: Detailed Explanation of Larval Stages:
The life cycle proceeds in the following order:
1. Miracidium: This is the first larval stage that hatches from the egg in freshwater. It is a free-swimming larva, but its motility is achieved through the beating of numerous cilia that cover its body, not a tail. Its goal is to find and penetrate a suitable snail host.
2. Sporocyst: Once inside the snail, the miracidium transforms into a sac-like sporocyst. This stage is non-motile and reproduces asexually to form either more sporocysts or the next stage, rediae.
3. Redia: This stage develops within the sporocyst. It has a mouth and a pharynx and is more developed than the sporocyst but is still non-motile within the snail's tissues. It also reproduces asexually to produce more rediae or cercariae.
4. Cercaria: This is the larval stage that emerges from the snail host back into the water. It is characterized by having a distinct body (or head) and a long, powerful tail. The tail is used for swimming, allowing the cercaria to move through the water to find and encyst on aquatic vegetation.
5. Metacercaria: The cercaria loses its tail and forms a protective cyst on vegetation, becoming a metacercaria, which is the infective stage for the final host (e.g., sheep, cattle, humans).

Step 3: Final Answer:
Among the options given, the Cercaria is the only larval form that possesses a distinct, motile tail for swimming. Therefore, option (D) is the correct answer.