Question

Which of the following have notched throughout life?
A. Birds
B. Fish
C. Snake
D. Amphioxus

Answer 1

Hint: Notochord refers to the rod-like structure developed on the dorsal side in some animals during embryonic development. In Vertebrates, the notochord gets transformed into the vertebral column.

Complete answer: The Notochord is a rod-shaped structure of large vacuolated cells lying along the mid-dorsal line of the body between the nerve cord and the digestive canal. It is derived from the cells in the mesoderm. At any developmental stage of their lives, chordates are distinguished by the presence of notochords. This is what differentiates between non-chordates and chordates. However, chordates are further categorized into vertebrates and invertebrates.
Vertebrates have notochord replaced by the vertebral column. It encases the spinal cord during the course of animal growth and development. Birds (Aves), fish (Pisces), and snakes (Reptiles) are also vertebrates. This renders it incorrect for choices A, B, and C.
Amphioxus is a non-vertebrate, yet a chordate animal.
It has its notochord throughout life, but a vertebral column or backbone is lacking in this animal.

So, the correct answer is “Option D”.

Note: Except Vertebrata, two more subphylas are there in the phylum Chordata, viz. Urochordata and Cephalochordata. These are sometimes referred to as “Protochordates”.
Amphioxus, the marine animals belong to Cephalochordata.
There is a notochord in them that stretches from the head to the tail area.
During the life of the organism, the structure remains persistent. It is flexible and quite similar to cartilage.