Question

The thyroid gland is homologous to which part of lower chordates?
A) Neural gland
B) Pharyngeal gill pouch
C) Nerve cord
D) Endostyle

Answer 1

Hint: Thyroid gland is a butterfly shaped gland which is situated at the base of the neck and is responsible for releasing the hormone that controls the metabolism of an individual.

Complete answer:
Thyroid gland in an endocrine that releases three hormones namely triiodothyronine (T3), thyroxine (T4) and calcitonin. The thyroid hormones are responsible for controlling metabolic rate and protein synthesis. Their secretion is regulated by Thyroid stimulating hormone (TSH) which is secreted by the anterior pituitary gland. In Chordates, certain glands are believed to perform a similar function to that of thyroid gland. Let us analyse the options to find out the correct option.

Option A: Neural gland is found in ascidians and is a glandular mass that is located near the nerve ganglion. It is homologous to the pituitary gland of the vertebrates.Therefore, this is the incorrect option.

Option B: Pharyngeal gill pouch are formed during embryogenesis in vertebrates on the endodermal side between the pharyngeal arches.Therefore, this is the incorrect option.

Option C: Nerve cord is a unique feature found in Chordates and is a site for attachment of the skeletal muscles which helps in coordinating the functions of the body.Therefore, this is the incorrect option.

Option D: Endostyle is a pair of longitudinal folds that project into the pharyngeal cavity of the Chordates like urochordates, cephalochordates and larvae of lampreys. It secretes mucus which chordates in trapping the food. Additionally, it also binds iodine to tyrosine just like the thyroid gland in vertebrates. This is usually their secondary function but becomes primary when larvae become an adult and a predator. Therefore, this is the correct option.

Thus, the correct answer is option (D) Endostyle.

Note: With the evolution process in Chordates the jaws and the predatory structures were gradually lost and this was replaced by the functions that are performed by the endocrine gland.