Chapter 27, Problem 21A

Summary Introduction

Introduction: Chordates are species belonging to the phylum Chordata that at some stage during their development have four distinctive characteristics-a dorsal tubular nerve cord, pharyngeal pouches, a notochord and a postanal tail.

Answer to Problem 21A

Correct answer:

The correct answer is option (A) 1

Explanation of Solution

Explanation/justification for the correct answer:

Option (A) 1. 1 represents notochord which is elastic, rod-like structure stretching the body circumference. An animal with a notochord can allow side-to - side motions of the body and tail, making fish-like swimming possible for the first time in the history of evolution. So, the correct answer is option (A).

Explanation for incorrect answer:

Option (B) 2. 2 represent dorsal tubular nerve cord. The nerve cords in nonchordates are ventral to and below the digestive structure, and are strong. Chordates do have dorsal tubular nerve cord that is situated in or above the digestive organs and is a form of a tube. So, this is an incorrect answer.

Option (C) 3. 3 represent pharyngeal slit. Pharyngeal pouches are the coupled structures connecting the muscle tube which connects the cavity of the mouth and the esophagus. It includes slits leading outwards. These structures were initially used for filter feeding, and later formed into gills for water gas exchange. So, this is an incorrect answer.

Option (D) 4. 4 represent thyroid gland. The thyroid gland is a system where metabolism, growth and development are regulated. So, this is an incorrect answer.