This is a characteristic feature of fishes

The correct option is "gills and venous heart".

To understand why this is the characteristic feature of fishes, let we break down the anatomical and physiological features of fish step-by-step:

1. Gills (Respiratory Organ):
Fishes are aquatic organisms that require a specialized system to extract dissolved oxygen from water. Gills are vascularized structures that facilitate gaseous exchange (absorbing oxygen and releasing carbon dioxide) as water passes over them. While some other aquatic organisms might have gills, they are a primary and defining respiratory characteristic of fishes.

2. Venous Heart (Circulatory System):
The heart of a fish is described as a "venous heart" (or a two-chambered heart consisting of one atrium and one ventricle). It is called a venous heart because only deoxygenated (venous) blood flows through it.
The pathway of circulation in a fish is single circulation:
Deoxygenated blood from the body tissues ⇒ Atrium ⇒ Ventricle ⇒ Gills (where it gets oxygenated) ⇒ Body tissues.
Since the heart only pumps deoxygenated blood, the presence of a venous heart is a unique and distinguishing characteristic of fishes.

Why other options are incorrect:
- Epidermal scales: The scales of fishes are typically dermal in origin (derived from the dermis layer of the skin), whereas epidermal scales are characteristic of reptiles.
- Tail: Many other vertebrates, such as amphibians, reptiles, birds, and mammals, also possess a tail, making it non-exclusive to fishes.

Therefore, the combination of gills and a venous heart serves as a definitive characteristic feature of fishes.