Guppy

The guppy ("Poecilia reticulata"), is a species of freshwater aquarium fish. It is one of the most popular aquarium fish species. The guppy is also called the millionfish or rainbow fish. It is a small member of the Poeciliidae family (females 4–6 centimeters (1.6–2.4 in) long, males 2.5–3.5 centimeters (1.0–1.4 in) long). Like all other members of the family, it is live-bearing. Guppies are popular aquarium kept fish because of their inexpensive needs and beautiful color ranges
Guppies, like all fish, have gills. The heart of a guppy is very different from that of a mammal, for instance there are two compartments instead of four. They are called the auricle and ventricle. When the fish breaths the ventricle contracts and blood is forced out to the gills which act as lungs. This is where the gas exchange happens. The blood receives oxygen and releases carbon dioxide. The blood that now carries oxygen goes through the dorsal artery, which divides into smaller and smaller vessels and go all over the body to then distribute the oxygen and gets carbon dioxide and waste. Then, the blood is carried to the heart by veins and lymph vessels. Then the cycle repeats over and over.
A guppy's nervous system is highly centralized around its spinal cord and brain. The cerebellum controls the body's movements, and the olfactory bulbs and cerebrum controls its sense of smell.