Tag: diversity in living world

Vertebrates are the subphylum of a Chordata. Placoderms were among the first jawed fish. The jaws are considered to evolve from the gill arches. Placoderms are the extinct class of armoured prehistoric fishes, known from fossils, which lived from the Silurian to the end of the Devonian period. Their head and thorax were covered by articulated armoured plates and the rest of the body was scaled or naked.

Cyclostomes are the organisms which belong to the group of chordates. These are the jawless fishes. The group also includes the lampreys and hagfishes. They have rounded mouths that lack jaws. The water keeps on cycling through the open mouth which cannot be closed due to lack of jaws. 

Carassius is a genus in the ray-finned fish family Cyprinidae.
Hippocampus is the name of marine fishes in the genus Hippocampus.
While Lepisma is a genus of primitive insects.
So, the correct answer is option D.

The hippocampus is the name of marine fishes.
Torpedo is the group of flattened cartilaginous fishes, also called an electric ray.
Mystus is a genus of small to medium-sized bagrid catfishes.
Petromyzon is the 'sea lamprey', which is parasitic lamprey. It is a 'jawless' fish.

In cyclostomes, the pineal gland is a photoreceptor. The endocrine nature of the gland's function has not been definitely established. As it's activity diminishes with age, the gland turns into rudimentary organ containing mineral deposits.

Cyclostomes are animals with a circular mouth, eel-shaped body with jaw less mouth, single median nostril and no paired fins. So, the correct answer is option B.

Myxine is a genus of hagfish, which are eel-shaped, slime-producing marine fish. 
Petromyzon is a parasitic lamprey found in the northern Atlantic ocean along shores of Antartica and North America, in the western Mediterranean sea and in the shores of the Great lakes. It is brown, gray, or black on its back and white or gray on the underside and can grow up to 90 cm long. 

Ammocoete larva is the larval stage of the primitive jawless fish, called as the lamprey or Petromyzon. Adult lampreys spawn in rivers and then die. The young larvae, ammocoetes, spend several years in the rivers, where they live burrowed in fine sediment, filter feeding on detritus and microorganisms. Then, ammocoetes undergo a metamorphosis lasting several months.