Which one feature is common to $Amphioxus$,frog,sea horse,and crocodile?

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:

Apart from Annelida and Arthropoda,metamerism is also exhibited by ............ .

Provide definitions/explanations for the following terms:
$(i)$ Cephalochordata
$(ii)$ Pseudocoelomates

Give three major differences between chordates and non-chordates and draw a schematic sketch of a chordate showing those features.

Which animal is included in the subphylum Cephalochordata?