Question

Which one is absent in Branchiostoma?
A) Head
B) Gill slits
C) Notochord
D) Dorsal hollow nerve cord

Answer 1

Although it is a chordate, it has no vertebral column and no bones at all, making it an invertebrate. It does have a pharynx that has developed as a feeding apparatus, which is done by sucking water into and then filtering it.

Complete answer:
Branchiostoma is a marine invertebrate also known as a lancet. It is often used to study the evolutionary development of vertebrates.
An unusual characteristic of this organism is that it is a chordate, and the notochord extends into what would be the head region, but there is no actual head. Option A is correct.
This unusual organism has gill slits on the pharynx which are a part of the feeding apparatus, filtering out food particles from the water. Option B is not correct.
Option C is incorrect. Branchiosotoma belongs to the phylum Chordata, so it has a notochord. This notochord is a hydrostatic organ, using water pressure to resist compression, and allowing flexion of the notochord and body.
Option D is incorrect. Cross sections of the organism, particularly in the oral region show the presence of a hollow notochord.
The reason this organism has been called Lancelet is because it is long and slender like a lancet.

Hence the correct answer is OPTION(A)

Note: The organism is used extensively as a laboratory research animal. Interestingly, though lacking a head and brain, it does have a cluster of light sensitive cells forming an extremely rudimentary eye. This will not actually see and process images, but it will be able to direct the organism away or towards light sources.