Question

A group of researchers procured some fish-like animals and upon investigation the following characters were observed:
A. Endoskeleton was made of cartilage.
B. Ectoparasitic; as they were found attached on fish skin with their circular sucking mouth.
C. Paired fins and scales are absent, but 7 pairs of gill slits were present.
Which of the following species of animals did they consider to fit best with these characters?

• A. Petromyzon sp.
• B. Branchiostoma sp.
• C. Scoliodon sp.
• D. Exocoetus sp.

Hint:

Logic Tip: The phrase "circular sucking mouth" is the most unique identifier here. It immediately points to Agnatha (jawless fishes) specifically the Cyclostomes like Lampreys ({Petromyzon}) and Hagfishes ({Myxine}).

Answer 1

The Correct Option is A

Step 1 : Understanding the Question:
The question provides a set of diagnostic physiological and anatomical features of a specific chordate. We must determine which of the listed animals belongs to the class that exhibits all these specific "primitive" or "jawless" traits.
Step 2 : Key Formulas and Approach:
The described traits (cartilaginous endoskeleton, circular sucking mouth, jawless, lack of scales/paired fins) are the defining characteristics of the Class Cyclostomata. We need to identify which genus listed belongs to this class.
Step 3 : Detailed Explanation:

Identifying the Class: The absence of jaws (Agnatha), paired fins, and scales, combined with a cartilaginous skeleton and a parasitic lifestyle involving a circular mouth, points directly to the cyclostomes.

Evaluating the Options:
Petromyzon (A): Commonly known as the lamprey. It is a cyclostome. It matches all the given traits: it is an ectoparasite on fishes, has 6-15 pairs of gill slits (7 pairs falls in this range), and has no jaws or scales.

Branchiostoma (B): A cephalochordate (Amphioxus). It is not a true vertebrate and does not have the described sucking mouth or cartilaginous skull.

Scoliodon (C): A cartilaginous fish (shark). While it has cartilage, it possesses jaws, paired fins, and placoid scales. It is not an ectoparasite.

Exocoetus (D): A bony fish (flying fish). It has a bony skeleton, scales, jaws, and fins.


Step 4 : Final Answer:
Based on the strict morphological description of jawless, parasitic vertebrates, {Petromyzon} is the correct fit. This leads us to option (A).