Chapter 28, Problem 13A

Summary Introduction

Introduction:

Fishes live in aquatic habitats like lakes, seas, rivers, ponds, etc. Some fishes live in complete darkness at the bottom of the deep ocean whereas others live in the freezing water of the Polar Regions and have special proteins in their blood to keep the blood from freezing. There are about 24,600 species of living fishes found.

Answer to Problem 13A

Correct answer:

The correct answer is option (A) ostracoderms.

Explanation of Solution

Explanation/justification for the correct answer:

Option (A) ostracoderms. Ostracoderms were the second group of fish to appear in the fossil record of the Ordovic period. Ostracoderms had head shields made of bone, a bony outer shell, and a combination of fins. The bone construction of ostracoderms was an evolutionary landmark in the development of vertebrates. Even though ostracoderms have been extinct, scientists hypothesize that modern fish share an ancestor with ostracoderms. So, the correct answer is option (A).

Explanation for incorrect answer:

Option (B) ray-finned fishes. Ray-finned fishes belong to the bony fishes. The most distinctive characteristic of the ray-finned fish is its name. The thin membranes of these fish fins are protected by long, spine-like rays. So, this is an incorrect option.

Option (C) coelacanths. Coelacanths are plump, large, lobe-finned fish. They can grow upto more than 2 meters and weigh about 90 kilograms. It is estimated that they will live for 60 years or more. Modern coelacanths look bigger than those found to be fossils. So, this is an incorrect answer.

Option (D) jawless fishes. Jawless fishes can penetrate the body of the fish through their mouth or the prey scrape and open the fish with tooth-like structures on their tongues. After eating the inside of the fish, the hagfish leaves just a sack of skin and bones. So, this is an incorrect answer.