Usually,chordates have red blood containing red blood corpuscles. The blood is red due to the presence of the following pigment:

Fishes,amphibians,reptiles,and birds are kept in the same

Given below are two statements: one is labelled as Assertion $(A)$ and the other is labelled as Reason $(R)$.
Assertion $(A):$ All vertebrates are chordates but all chordates are not vertebrates.
Reason $(R):$ The members of subphylum Vertebrata possess a notochord during the embryonic period; the notochord is replaced by a cartilaginous or bony vertebral column in adults.
In the light of the above statements,choose the correct answer from the options given below:

The animal group,where the adults are degenerated but larvae are well developed,is

Which of the following animals is not a vertebrate?

The most important distinctive character of $Chordata$ is the presence of