An elongated marine vertebrate 'X' is found acting as an ectoparasite on some fishes. It possesses a closed circulatory system and a cartilaginous cranium and vertebral column. Which of the following represents a correct characteristic of animal 'X'?

Step-by-step solution

Based on the given characteristics (elongated marine vertebrate, ectoparasite on fishes, cartilaginous skeleton, closed circulation), animal 'X' belongs to Class Cyclostomata. Cyclostomes are marine but migrate to fresh water for spawning (anadromous migration). After spawning, they die within a few days. Their larvae, after metamorphosis, return to the ocean. Thus, the statement regarding larval return is correct. The other options are incorrect because Cyclostomes are devoid of scales and paired fins (placoid sca