Study the list of organisms below and use them to answer the questions. I-Hydra II-Owl III-Snake IV-Snail The…

Analyze the question: You are asked to arrange Hydra, Owl, Snake and Snail from the simplest to the most complex organism. This depends on their level of body organization and their evolutionary position.

Concept: Animals become more complex as you move from simple body plans to more advanced ones:

• Cnidarians (e.g. Hydra) have only two cell layers and a simple sac-like body with no true organs.
• Molluscs (e.g. Snail) have three cell layers, a true body cavity, organs, a muscular foot and a shell.
• Reptiles (e.g. Snake) are vertebrates, with a backbone, a well-developed brain and lungs.
• Birds (e.g. Owl) are advanced vertebrates. They are warm-blooded (endothermic), with a four-chambered heart, a high metabolic rate and highly developed senses and flight.

Step by step:

1. Hydra (I) is a Cnidarian, so it is the simplest.
2. Snail (IV) is a Mollusc, an invertebrate with organ systems, so it comes next.
3. Snake (III) is a Reptile, a cold-blooded vertebrate, so it is more complex than the snail.
4. Owl (II) is a Bird, a warm-blooded vertebrate, so it is the most complex.

Order of increasing complexity: Hydra → Snail → Snake → Owl, which is I – IV – III – II.

Checking the options:

• A starts with the owl, which is wrong.
• B is I-IV-III-II, which matches.
• C and D do not fit the sequence.

Correct answer: B

Exam tip: Remember the ladder as "Cnidarian → Mollusc → Reptile → Bird". Always move from invertebrates to vertebrates, and from cold-blooded to warm-blooded.