Biology Notes for IGCSE 2014 & 2024: 1.4. Dichotomous keys - Free Printable

Let's solve the problem step by step using the dichotomous key provided in the image. We need to identify each of the four animals (A, B, C, D) based on their physical characteristics and follow the key's logic.

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Step-by-step identification using the dichotomous key:

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#### Animal A (Crocodile-like animal)
Already identified as:
1b, 2a, 3a – *Crocodylus niloticus*

Let’s verify this:

- 1a: Shell present? → No → go to 2 ✔
- 2a: Four legs? → Yes → go to 3 ✔
- 3a: Back and tail covered with rough spikes? → Yes → Crocodylus niloticus ✔

✔️ Correct.

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#### Animal B (Turtle)

Look at the animal: it has a shell, is slow-moving, and has four legs.

Follow the key:

- 1a: Shell present? → Yes → Geochelone elephantopus ✔

So:
> 1a – *Geochelone elephantopus*

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#### Animal C (Snake-like animal)

This animal has no shell, no legs, and looks like a snake.

Follow the key:

- 1a: Shell present? → No → go to 2
- 2a: Four legs? → No → so 2b: Shell absent → Ophiophagus hannah ✔

Note: Ophiophagus hannah is the king cobra, a large venomous snake.

So:
> 1b, 2b – *Ophiophagus hannah*

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#### Animal D (Chameleon)

This animal has four legs, no shell, and a curled tail with a patterned body.

Follow the key:

- 1a: Shell present? → No → go to 2
- 2a: Four legs? → Yes → go to 3
- 3a: Back and tail covered with rough spikes? → No → so 3b: No spikes on tail → Chamaeleo gracilis ✔

So:
> 1b, 2a, 3b – *Chamaeleo gracilis*

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✔ Final Answers:

A: 1b, 2a, 3a – *Crocodylus niloticus*
B: 1a – *Geochelone elephantopus*
C: 1b, 2b – *Ophiophagus hannah*
D: 1b, 2a, 3b – *Chamaeleo gracilis*

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📝 Explanation Summary:

We used the dichotomous key, which presents pairs of contrasting statements (e.g., "shell present" vs. "shell absent"). At each step, we choose the option that matches the animal's features and proceed accordingly until we reach a species name.

Key observations:
- Shell is a major distinguishing trait (present in turtles).
- Legs help differentiate between snakes and legged reptiles.
- Spikes on back/tail distinguish crocodiles from chameleons.

This method ensures accurate identification by systematically eliminating possibilities.

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✔ All animals correctly identified using the key.