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

Concept:
The animal kingdom is divided into various phyla and classes based on specific morphological and anatomical features. The given characteristics point towards a jawless vertebrate belonging to the class Cyclostomata within the subphylum Vertebrata.
Step 1:

• A. Endoskeleton made of cartilage: This eliminates bony fishes (Osteichthyes).
• B. Ectoparasitic with circular sucking mouth: This is a defining feature of jawless fishes (Agnatha), which lack jaws and attach to hosts to suck blood.
• C. Paired fins and scales absent, 7 pairs of gill slits: The absence of paired fins and scales, along with specific gill slit numbers (usually 6-15 pairs), further confirms it is a cyclostome.
Step 2:
Petromyzon is commonly known as the lamprey. It belongs to the class Cyclostomata. Lampreys are jawless, possess a cartilaginous endoskeleton, lack scales and paired fins, have 6-15 pairs of gill slits for respiration, and many species are ectoparasites on other fishes, attaching with their circular, sucking mouth. This matches all given characteristics perfectly.
Step 3:
Branchiostoma (Amphioxus) belongs to the subphylum Cephalochordata. It is a small, fish-like filter feeder, not an ectoparasite. It does not have a distinct cartilaginous skull or the described sucking mouth.
Step 4:
Scoliodon is a cartilaginous fish (Class Chondrichthyes), commonly known as a dogfish shark. While it has a cartilaginous skeleton, it possesses jaws, paired fins (pectoral and pelvic), and placoid scales. It is a predator, not a sucking ectoparasite.
Step 5:
Exocoetus is a bony fish (Class Osteichthyes), commonly known as a flying fish. It has a bony skeleton, jaws, paired fins (highly modified pectorals), and scales.
Step 6:
Based on the analysis, only Petromyzon fits all the provided characteristics.