Entamoeba differs from amoeba in not having

The correct option is Contractile vacuole.

To understand why Entamoeba differs from Amoeba in not having a contractile vacuole, let us examine their respective habitats and cellular adaptations:

1. Habitat and Osmoregulation:
- Amoeba proteus is a free-living freshwater organism. Because the surrounding freshwater environment is hypotonic relative to the inside of the cell, water constantly enters the cell via osmosis. To prevent the cell from swelling and bursting, Amoeba uses a contractile vacuole to collect excess water and pump it out of the cell (a process called osmoregulation).
- Entamoeba histolytica, on the other hand, is an endoparasite living inside the human intestine. The osmotic concentration of the intestinal fluid is isotonic to the cytoplasm of Entamoeba. As there is no net influx of water into the cell, there is no need for osmoregulation. Consequently, Entamoeba lacks a contractile vacuole.

2. Shared Features:
Both Entamoeba and Amoeba belong to the same protozoan group and share several anatomical features:
- Pseudopodia: Used by both organisms for movement and capturing food (though they differ slightly in shape and structure).
- Nucleus: As eukaryotic organisms, both possess a well-defined nucleus containing their genetic material.
- Ectoplasm: The cytoplasm of both organisms is divided into an outer gel-like ectoplasm and an inner fluid-like endoplasm.

Therefore, the absence of a contractile vacuole in Entamoeba is the key distinguishing feature, adapted to its parasitic lifecycle in an isotonic environment.