In animal biology, which of the following animals is known to have hard structures that act like teeth inside its stomach to grind food?

Introduction / Context:
Different animals have evolved very specialised digestive systems depending on what they eat and how they capture food. Some have sharp teeth in the mouth, some swallow food whole, and a few species even have grinding structures inside the digestive tract itself. This question focuses on an interesting fact from zoology: which animal is described as having teeth in the stomach.

Given Data / Assumptions:

• Options include frogs, crabs, snakes, cockroaches and earthworms.

• The phrase teeth in the stomach refers to internal grinding structures, not actual teeth in the mouth.

• The learner is expected to recall special adaptations in the digestive systems of invertebrates.

• General school level zoology facts are assumed.

Concept / Approach:
Many crustaceans, including crabs, possess a structure called the gastric mill within their stomach. The gastric mill contains hard chitinous plates that function like teeth and help grind food after it has been swallowed. This adaptation is necessary because crabs often consume hard food items such as shells and plant material. The other animals listed either swallow food whole or use mouthparts and do not have stomach teeth in this sense.

Step-by-Step Solution:
Step 1: Interpret the phrase teeth in the stomach as referring to internal grinding plates or structures. Step 2: Recall that crustaceans such as crabs and lobsters are known to have a gastric mill with hard ridges acting like teeth. Step 3: Check the options to see which animal belongs to the crustacean group. Step 4: Identify crabs as crustaceans, while frogs, snakes, cockroaches and earthworms belong to different taxa. Step 5: Select crabs as the correct answer.

Verification / Alternative check:
Basic zoology texts and animal biology references describe the internal anatomy of crustaceans, including the presence of a gastric mill in the cardiac stomach region. Diagrams of crab or lobster digestive systems label these chitinous grinding plates and explicitly compare them to teeth. In contrast, standard descriptions of frogs, snakes, cockroaches and earthworms do not mention any teeth inside the stomach; they rely on other methods for mechanical digestion, such as gizzards in birds or muscular grinding in earthworms.

Why Other Options Are Wrong:
Frogs use a simple digestive tract and swallow food more or less whole, relying on stomach acids and enzymes for digestion, not stomach teeth. Snakes also swallow prey whole and use strong digestive juices instead of internal grinding plates. Cockroaches have chewing mouthparts and a crop and gizzard region, but the phrase teeth in the stomach is classically associated with the gastric mill of crustaceans. Earthworms possess a gizzard with muscular grinding but not hard teeth like structures inside the stomach. Therefore, these choices do not match the specific zoological fact being tested.

Common Pitfalls:
Learners sometimes confuse gizzards and grinding structures in birds or earthworms with the term stomach teeth and may guess wrongly. Another pitfall is to focus on animals that swallow prey whole, such as snakes, and imagine that they must have special internal teeth. To avoid such errors, it is useful to remember that the phrase teeth in the stomach is a classic exam line for crabs and other crustaceans that have a gastric mill.

Final Answer:
The animal known to have teeth like grinding structures inside its stomach is the crab.