Chapter 24, Problem 32A

Summary Introduction

Introduction :

Sponges and cnidarians were the first animals to evolve from a multicellular origin. Most sponges and cnidarians live in marine water. Sponges have no tissues, no organs, and most have no symmetry. Most cnidarians have 2 layers of cells and one opening of the body. Such two layers are divided into tissues with different functions.

Answer to Problem 32A

Correct answer :

The correct answer is option (D) cnidarians - spicules.

Explanation of Solution

Explanation/justification for the correct answer :

Option (D) cnidarians - spicules. Cnidarians do not have any spicules. Spicules have been found in the sponge. Spicules are the support structures for the sponges. They are a thin, needle-like structure made up of calcium carbonate, spongin and silica. So, the correct answer is option (D).

Explanation for incorrect answer :

Option (A) sponges - filter feeding. Sponge gets its food by filtering small particles from water. This process of filtering is known as filter feeding. It is one of the main characteristic of sponge. So, this is an incorrect option.

Option (B) cnidarians - nematocysts. Cnidarians have cnidocytes that contain nematocyst. Nematocysts are the capsules that hold coiled, thread-like tube containing barbs and poison. So, this is an incorrect answer.

Option (C) sponges - free swimming larva. Sponges are reproduced by external fertilization. After fertilization, the zygote develops into a larva that is free to swim and has flagella. The larva finally sticks to the surface and transforms into an adult sponge. So, this is an incorrect answer.