The body is often fusiform, a streamlined body plan often found in fast-moving fish. At the broadest level, their body is divided into head, trunk, and tail, although the divisions are not always externally visible. įish have a variety of different body plans. However, it still shares the same basic body plan from which all vertebrates have evolved: a notochord, rudimentary vertebrae, and a well-defined head and tail. In many respects, fish anatomy is different from mammalian anatomy. They mostly spawn a large number of small eggs with little yolk which they broadcast into the water column. Bony fish have a swim bladder which helps them maintain a constant depth in the water column, but not a cloaca. The dermis is covered with overlapping scales. They have a bony skeleton, are generally laterally flattened, have five pairs of gills protected by an operculum, and a mouth at or near the tip of the snout. ![]() The bony fish lineage shows more derived anatomical traits, often with major evolutionary changes from the features of ancient fish. Some species are ovoviviparous, having the young develop internally, but others are oviparous and the larvae develop externally in egg cases. Cartilaginous fish produce a small number of large yolky eggs. They have a cloaca into which the urinary and genital passages open, but not a swim bladder. The dermis is covered with separate dermal placoid scales. Their bodies tend to be dorso-ventrally flattened, and they usually have five pairs of gill slits and a large mouth set on the underside of the head. Sharks and rays are basal fish with numerous primitive anatomical features similar to those of ancient fish, including skeletons composed of cartilage. Low-frequency vibrations are detected by the lateral line system of sense organs that run along the length of the sides of fish, which responds to nearby movements and to changes in water pressure. There is an inner ear but no external or middle ear. The eyes are adapted for seeing underwater and have only local vision. The heart has two chambers and pumps the blood through the respiratory surfaces of the gills and then around the body in a single circulatory loop. They are supported by the muscles which compose the main part of the trunk. The main external features of the fish, the fins, are composed of either bony or soft spines called rays which, with the exception of the caudal fins, have no direct connection with the spine. The ribs attach to the spine and there are no limbs or limb girdles. The main skeletal element is the vertebral column, composed of articulating vertebrae which are lightweight yet strong. The skeleton, which forms the support structure inside the fish, is either made of cartilage ( cartilaginous fish) or bone ( bony fish). The body of a fish is divided into a head, trunk and tail, although the divisions between the three are not always externally visible. Water is much denser than air, holds a relatively small amount of dissolved oxygen, and absorbs more light than air does. The anatomy of fish is often shaped by the physical characteristics of water, the medium in which fish live. In practice, fish anatomy and fish physiology complement each other, the former dealing with the structure of a fish, its organs or component parts and how they are put together, such as might be observed on the dissecting table or under the microscope, and the latter dealing with how those components function together in living fish. It can be contrasted with fish physiology, which is the study of how the component parts of fish function together in the living fish. Fish anatomy is the study of the form or morphology of fish.
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