Bilateral symmetry,cylindrical bodies and double openings are characteristic features of

Analyze the question: We need the group of organisms with three features: bilateral symmetry, a cylindrical body, and "double openings" (a complete digestive tract with a mouth and an anus).

Explain the concept:

• Nematodes (roundworms) are bilaterally symmetrical and have long, cylindrical, unsegmented bodies tapered at both ends. They have a complete alimentary canal, with a mouth at one end and an anus at the other. Examples are Ascaris and hookworm.
• Hydra belongs to Cnidaria. It has radial symmetry and a sac-like gut with only one opening, which serves as both mouth and anus.
• Protozoa are single-celled organisms. They have no true body symmetry in the animal sense and no mouth/anus system. Some have a cell mouth (cytostome).
• Protists are a broad group of mostly unicellular eukaryotes and do not have these features.

Solution:

| Feature | Nematodes | Hydra | Protozoa | Protists | |---|---|---|---|---| | Bilateral symmetry | ✔ | ✘ (radial) | ✘ | ✘ | | Cylindrical body | ✔ | Partly | ✘ | ✘ | | Two openings (mouth and anus) | ✔ | ✘ (one) | ✘ | ✘ |

Only nematodes match all three features.

Correct answer: A. nematodes

Tip: Remember "Nematodes = Nice round tubes." The name comes from the Greek nema (thread), so think of thread-like, round worms with a through-gut (mouth → anus). Hydra and other cnidarians are radial with one opening, so you can eliminate them quickly.