Chapter 27, Problem 26A

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 26A

Correct answer:

The correct answer is option (B) endostyle

Explanation of Solution

Explanation/justification for the correct answer:

Option (B) endostyle. The thyroid gland is a system where metabolism, growth and development are regulated. An early type of a thyroid gland originated in early chordate cells which secreted mucus as a help for filter feeding. Invertebrate chordates in this same region have endostyle glands that produce proteins identical to those secreted by the thyroid gland. So, the correct answer is option (B).

Explanation for incorrect answer:

Option (A) 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) notochord. The notochord is an elastic, rod-like structure, extending the body length. It lies just below the nerve cord of the dorsal tubular. The notochord is replaced by bone or cartilage in most vertebrate animals. The notochord persists in the chordate of vertebrate individuals. So, this is an incorrect answer.

Option (D) pharyngeal pouches. 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.