Question 29

Reptiles are characterized by bilateral symmetry and dependence on external heat sources for regulating body temperature.

Reptiles exhibit bilateral symmetry, which means their body structure can be divided into mirror-image halves along one plane. Additionally, they are ectothermic, relying on external environmental temperatures to regulate their body heat.

A) Insects

Insects do display bilateral symmetry, but they are not dependent on external heat sources for temperature regulation in the same way reptiles are. Insects can exhibit behavioral adaptations to manage their body temperature; however, their metabolic processes are not as reliant on environmental heat as those of reptiles.

B) Sponges

Sponges do not possess bilateral symmetry; instead, they have an asymmetrical or radial symmetry. Furthermore, sponges are sessile and do not regulate body temperature, as they lack a complex nervous system and do not require temperature regulation like ectothermic organisms.

C) Mammals

Mammals are characterized by bilateral symmetry, but they are endothermic rather than ectothermic. This means they generate their own body heat internally and do not rely on external heat sources for temperature regulation, which disqualifies them from the question's criteria.

D) Reptiles

Reptiles are indeed characterized by bilateral symmetry, and they are ectothermic, meaning they depend on external heat sources to maintain their body temperature. This makes them the correct choice for the question, as they perfectly fit the description provided.

Conclusion

Reptiles are the only organisms listed that meet both criteria of bilateral symmetry and dependence on external heat sources for temperature regulation. Insects and mammals, while they exhibit bilateral symmetry, do not rely on external heat sources in the same way. Sponges do not possess bilateral symmetry and lack the necessary systems for temperature regulation, further confirming that reptiles are the definitive answer.