I couldn't find any threads on here about betta anatomy and it seems like common questions occur when people ask what are ventrals or what is the caudal. Maybe this can clear it up a little bit! :)
All credits go to bettacare101.com!
The dorsal fin is located on the top of the fish and is used for stability when swimming through the water so that your betta can swim in a straigth line. Without it, your betta would stuggle to swim straight and have a hard time getting to where it needs to go.
The caudal fin in bettas come in many shapes which define each type of betta such as halfmoons, crowntails, plakats, etc. They also provide the power and sudden bursts, and act as a rudder for turning. However, the lengthening of the caudal fins in bettas causes them to be slow swimmers and the large fins act as parachutes in that they easily catch the water currents from filters and the betta gets pushed around the tank.
The anal fin is located on the bottom side of the betta between the pelvic (ventral) fins and the caudal fin. Similar to the dorsal fin, the anal fin provides stability when the betta swims through the water.
The pectoral fins are located on each side of the betta are responisble for turning and navigation.
The pelvic fins are located between the anal fins and the mouth, one on each side of the betta and are used for steering.
Although bettas have developed the labyrinth organ that allows them to breathe from the surface, the still use their gills, especially when sleeping. In waters with a low oxygen concentration, bettas sleep near the surface to easily get breathe.
Bettas, as with other fish are able to see in two directions but cannot focus on one object at the same time. They also are nearsighted and can only see clearly objects up to a foot away. Additionally, because fish do not have eyelids, they sleep with their eyes wide open.
Bettas are a unique fish because of this special organ they developed. The labyrinth is located behind the gills and is filled with oxygen-absorbing blood vessels. This special organ allows bettas to directly take oxygen from the air, which is why many people think that it is okay to keep them in small tanks. However, bettas still produce the same amount of waste as other fish and need plenty of space to flourish.
The swim bladder is a very important fish organ in that it allows fish to stay in an upright position and swim to varying depths in the aquarium. Without a swim bladder, your betta would sink to the bottom. It is filled with gas and the amount of gas is changed based on where the betta wants to move to.