Q.Which group of chordates possess sucking and circular mouth without jaws?

Class Cyclostomata possesses a sucking, circular mouth without jaws.

• Cyclostomes are the most primitive living vertebrates, and their mouth is circular and adapted for sucking, entirely lacking jaws.
• This jawless condition places them in the division Agnatha, separate from all jawed vertebrates (Gnathostomata). …

Among chordates, the presence or absence of jaws marks a major evolutionary divide, splitting the vertebrates into the jawless Agnatha and the jawed Gnathostomata.

• Cyclostomes have an elongated body and a mouth that is circular and adapted for sucking, with no jaws at all — this jawless condition is exactly why they are placed in the division Agnatha.
• They live as ectoparasites, attaching to the outside of certain fishes using this sucking mouth.
• Respiration occurs through 6 to 15 pairs of gill slits, the skeleton (cranium and vertebral column) is entirely cartilaginous, and the circulatory system is closed. …

Method: Binary-split elimination (jawed vs jawless)

Rather than recalling Cyclostomata directly, use the single biggest split within the chordates — jawed versus jawless — as a first filter, and only then look inside the jawless branch for the specific mouth type described.

Step 1 — apply the first, biggest split. All chordates with a vertebral column divide into two groups: Gnathostomata (jaw-bearing) and Agnatha (jawless). A "sucking and circular mouth without jaws" is, by definition, a jawless-branch feature, so the entire Gnathostomata branch — which includes Pisces, Amphibia, Reptilia, Aves and Mammalia — can be set aside immediately without checking any of them individually.

Step 2 — look inside the remaining (Agnatha) branch. Among living vertebrates, Agnatha is represented by a single class: Cyclostomata. This class is described as the most primitive living vertebrates, with an elongated body and a mouth that is specifically circular and adapted for sucking, entirely lacking jaws — exactly matching the description in the question. …