Question

Petromyzon and Myxine are not regarded as true fishes, because
A. They lack unpaired fins
B. They have circular mouth
C. In them operculum is absent
D. There is absence of paired fins and jaws

Answer 1

Hint: Fishes are the gill-bearing aquatic living animal. They are a vertebrate that belongs to the phylum Chordata. Fishes are found in both fresh and saltwater. Approximately 34,000 species of fishes have been found.

Complete answer:
Fish are aquatic vertebrate animals that belong to the phylum Chordata which can habitat in both the fresh and salt waters of the world. Generally, the fish habitats in freshwater and the body are covered with scales. Most fish species hold the characteristic of being cold-blooded and they are unable to regulate their body temperatures. For that, they depend on the change in temperature of the environment.
Fishes bear gills for the purpose of breathing by absorbing oxygen from the water. The swim bladder is present in all fishes, an organ that helps to maintain the buoyancy of fishes in the water. The presence of paired fins is the most common feature of the true fishes, which is responsible for their movement underwater. Jaws are present in complete fish.
Myxine belongs to the genus of hagfish and belongs to the class Cyclostomata. It has a shape like an eel. This is a slime-producing marine fish. Petromyzon is a type of parasitic lamprey which also belongs to the class Cyclostomata. This species is commonly found in the northern Atlantic Ocean along shores of Antarctica and North America. They also occur in the areas of western Mediterranean Sea. They can be grown up to 90 cm long and are grey, brown or even black in colour. These animals are representatives of ancient early fish like creatures and are the only living species. Like the other species that belong to class Cyclostomata, these species also do not have paired fins and jaws. Hence, they are not regarded as true fish.
Thus, the right option is D.

Note: Fish are aquatic, gill-bearing organisms. They have some special characteristics such as they are cold-blooded, presence of scales, and swim bladder. They also possess paired fins and jaws. In the class of Cyclostomata, the species does not have any paired fins. Jaws are also absent in the species which belong to this class. Petromyzon and Myxine belong to the class Cyclostomata and they also don’t have paired fins and jaws.