Oct 16, 2026 · 14 min · 11 segments
With the cartilaginous fish covered, let's move on to the bony fish, Osteichthyes. This is split up into Actinopterygii and Sarcopterygii. The first of these clades is very large, containing most of…
So let's examine the external anatomy of a typical bony fish, and we can start by first reviewing the planes of symmetry, since this will help in understanding their anatomy.
On this image, the sagittal plane separates the fish bilaterally, creating two mirror images.
The transverse plane separates the cranial end, the end with the head, from the caudal end, the end with the tail.
In most species of fish, the cranial region is also the anterior region, and the caudal region is also the posterior region.
The frontal plane separates the dorsal end, the back, from the ventral end, the stomach.
The eyes and nares, which lead to the olfactory organs, are separated by the sagittal plane, which divides the mouth.
The lateral line runs the length of the body and allows bony fish to detect movement in the water.
The dorsal fin helps with stabilization and is typically located on the midline of the fish's back and may be divided into two or more sections.
The pectoral fins are located on each side of the fish's body and are used mainly for steering and braking, while the pelvic fins are located on the ventral part of the fish and aid in movement and maneuverability.
The anal fin is located on the ventral surface of a fish, posterior to the anus, and helps to maintain stability and control during swimming by providing lift and aiding in maneuverability.
Finally, the caudal fin, or tail fin, is one of the most important fins for movement, as it propels the fish forward through the water by moving from side to side.
The shape and size of the caudal fin can vary between different species of bony fish, depending on their swimming abilities and behaviors.
For example, the homocercal, or equal-lobed caudal fin, is symmetrical, has the same shape on the upper and lower lobes, and is most common in bony fish.
The heterocercal caudal, or unequal-lobed fin, is asymmetrical and common in cartilaginous fish.
So let's examine the external anatomy of a typical bony fish, and we can start by first reviewing the planes of symmetry, since this will help in understanding their anatomy.
On this image, the sagittal plane separates the fish bilaterally, creating two mirror images.
The transverse plane separates the cranial end, the end with the head, from the caudal end, the end with the tail.
In most species of fish, the cranial region is also the anterior region, and the caudal region is also the posterior region.
The frontal plane separates the dorsal end, the back, from the ventral end, the stomach.
The eyes and nares, which lead to the olfactory organs, are separated by the sagittal plane, which divides the mouth.
The lateral line runs the length of the body and allows bony fish to detect movement in the water.
The dorsal fin helps with stabilization and is typically located on the midline of the fish's back and may be divided into two or more sections.
The pectoral fins are located on each side of the fish's body and are used mainly for steering and braking, while the pelvic fins are located on the ventral part of the fish and aid in movement and maneuverability.
The anal fin is located on the ventral surface of a fish, posterior to the anus, and helps to maintain stability and control during swimming by providing lift and aiding in maneuverability.
Finally, the caudal fin, or tail fin, is one of the most important fins for movement, as it propels the fish forward through the water by moving from side to side.
The shape and size of the caudal fin can vary between different species of bony fish, depending on their swimming abilities and behaviors.
For example, the homocercal, or equal-lobed caudal fin, is symmetrical, has the same shape on the upper and lower lobes, and is most common in bony fish.
The heterocercal caudal, or unequal-lobed fin, is asymmetrical and common in cartilaginous fish.
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