Fishes have two chambered heart one auricle and one ventricle, which receive only venous blood and pump it to gills for purification

Pisces, amphibia and reptiles are unable to maintain constant body temperature hence, called poikilothermic or cold blooded animal

Sea horse (Hippocampus) belongs to the class osteichthyes (due to bony skeleton) of super class pisces.

One auricle and one ventricle.

Elasmobranchii(dog fish) is an alernative name for cartilaginous fish or chondrichthyes. The name refers to the fact that the gill-slits are exposed and not covered by an operculum.

Placoid scale has a disc like basal plate. It resembles a tooth. These scales are found in cartilaginous fishes(chondricthyes) –

Cyclostomata is a class of group Agnatha, where paired appendages girdles and jaw are absent. Some workers include even the protochordates in Agnatha.