Problem 13 - Entrance Test

Which of the following combinations of characteristics is unique to Cyclostomes within the Chordata, distinguishing them from other jawed vertebrates?

A. Presence of notochord throughout life, two-chambered heart, and external fertilization.B. Absence of jaws, parasitic on other fishes, paired fins absent, and scale-less skin.C. Cartilaginous endoskeleton, oviparous, and presence of a lateral line system.D. Gills for respiration, distinct brain, and a closed circulatory system.

Correct: B

Cyclostomes (lampreys and hagfishes) are primitive jawless vertebrates. Let's analyze the options: (A) Notochord throughout life: True for some cyclostomes and some other primitive chordates, but not unique to them among all vertebrates. Two-chambered heart and external fertilization are common in many fishes. Not unique combination. (B) Absence of jaws: A defining feature (agnatha). Parasitic on other fishes: Characteristic of lampreys. Paired fins absent: True for cyclostomes. Scale-less skin: True for cyclostomes. This combination of features is unique to cyclostomes, setting them apart from all jawed vertebrates (Gnathostomata). This statement is CORRECT. (C) Cartilaginous endoskeleton: True for cyclostomes and Chondrichthyes (cartilaginous fishes). Oviparous: While some cyclostomes are oviparous, this is not unique. Lateral line system: Present in cyclostomes and most fishes. Not a unique combination distinguishing them from all jawed vertebrates. (D) Gills for respiration, distinct brain, and a closed circulatory system are features common to most vertebrates, including many jawed fishes. Not unique to cyclostomes.