Chapter 27, Problem 8STP

Summary Introduction

Introduction:

Flatworms belong to the phylum Platyhelminthes. They have a bilaterally symmetrical body and have a central nervous system. They are acoelomate, that is, they do not have a body cavity. They have three layers in their body. They also possess an incomplete digestive system.

Answer to Problem 8STP

Correct answer:

The correct answer is option (A) bilateral symmetry.

Explanation of Solution

Explanation/justification for the correct answer:

Option (A) bilateral symmetry. Flatworms have a bilaterally symmetrical body. The first body structure that appeared with the evolution of flatworms is their bilaterally symmetrical body. So, this is the correct answer.

Explanation for incorrect answer:

Option (B) coelomic cavity. The coelomic cavity is not present in the flatworms. Therefore, they are also known as acoelomates. So, this is an incorrect option.

Option (C) nervous system. Flatworms consist of a central nervous system. They are the first animals to develop a central nervous system along with a head that consists of a brain. But the first body structure that appeared in the flatworms was its bilaterally symmetrical body. So, this is an incorrect option.

Option (D) radial symmetry. Flatworms have a bilaterally symmetrical body. They do not possess radial symmetry. So, this is an incorrect answer.