Question

In hemichordates, the dorsal nerve cord is
A. Absent
B. Present throughout
C. Restricted to tongue region
D. Restricted to collar region

Answer 1

Hint: The presence of notochord is the characteristic feature of chordates. A phylum named Hemichordata includes the organisms which have a false notochord. The term ‘Hemichordate’ means ’Half-chordate’. This implies that a nerve cord is present and since it specifies ‘half’, it is present in a particular region and not throughout.

Complete answer:
Hemichordates are sometimes referred to as connecting links between chordates and non chordates. > Several features of the hemichordates are listed below :
> Some species of hemichordata are solitary while some live in colonies.
> They have a worm-like body and they are triploblastic.
> The hemichordates are also known as acorn worms due to the presence of proboscis.
> Bilateral symmetry and coelom is found in these animals.
> Their body is soft and consists of a single-layered epidermis.
> The body is divided into 3 regions – proboscis, collar, and trunk.
> The digestive tract is complete.
> The circulatory system is simple and well developed.
> In hemichordates, the dorsal nerve cord is present and it is restricted to the collar region of the body. It is not present throughout the body.
> Hemichordates undergo sexual reproduction. Fertilization is mostly external.

The correct answer is (D) Restricted to collar region.

Note: There is a key difference between nerve cord and notochord. The Notochord is a part of the skeletal system (rod-like structure) and the nerve cord is part of the nervous system (nervous tissue). Both are present in the organisms belonging to Phylum Chordata. So, the Phylum Hemichordata was introduced because the organisms belonging to this phylum showed few chordate characteristics and did not possess a notochord.