Which of these invertebrates possess a closed circulatory system?

Which among them is a characteristic of Chordata?

What do $a$ and $b$ represent in the given chart respectively?

Which of the following groups of animals have a closed circulatory system?

Which of the following statements are true for the phylum $Chordata$?
$(a)$ In $Urochordata$,the notochord extends from head to tail and is present throughout their life.
$(b)$ In $Vertebrata$,the notochord is present during the embryonic period only.
$(c)$ The central nervous system is dorsal and hollow.
$(d)$ $Chordata$ is divided into $3$ subphyla: $Hemichordata$,$Tunicata$,and $Cephalochordata$.

What are the general characteristics of the subphylum Vertebrata?