Chapter 24, Problem 28A

Summary Introduction

Introduction :

Cnidarians and sponges were the first animals that evolved from a multicellular ancestor. Most sponges live in marine water. Sponges have no tissues, no organs, and most have no symmetry. Sponges belong to the phylum Porifera that contains between 5000 to 10000 members.

Answer to Problem 28A

Correct answer :

The correct answer is option (D) asymmetry.

Explanation of Solution

Explanation/justification for the correct answer :

Option (D) Sponges show asymmetrical appearance. When the body is not divided into equal halves by any plane then it is called asymmetrical or asymmetric as found in some sponges. It is one of the characteristics of sponge. So, the correct answer is option (D).

Explanation for incorrect answer :

Option (A) cephalization. Differentiation of a definite head at the anterior end is termed as cephalization. Cephalization is not a characteristic feature of sponges. So, this is an incorrect option.

Option (B) cnidocytes. Cnidocytes is the characteristic features of cnidarians. These specialized cells are only found in cnidarians. So, this is an incorrect answer.

Option (C) bilateral symmetry. In this type of body symmetry, the body can be divided into two equal halves by a single plane only because of the important body organs are paired and occur on the two sides of a central axis. The given diagram shows asymmetrical appearance. So, this is an incorrect answer.