Dissection of a Bluespotted Flathead

This gallery aims to help students become familiar with the internal organs of a fish - the Bluespotted Flathead, Platycephalus caeruleopunctatus.

The body cavity and internal organs of a Bluespotted Flathead.

Notes

Once you have opened the body cavity, draw the parts you can see.

Add labels if you can identify the internal organs (parts).

Some of the internal organs of the Bluespotted Flathead.  The liver is the large orange organ on the left of the image.

The Bluespotted Flathead's heart is the pink structure in the centre of the image.

Notes

The heart is positioned closer to the head than the digestive organs.

Heart

The circulatory system in fishes is a single circuit, with blood flowing from the heart to the gills and then to the rest of the body. The heart is located a little behind and below the gills.

The typical fish heart has four chambers, however unlike mammals, blood moves through all four in sequence. Venous blood enters the sinus venosus (a thin walled sac) then flows into the atrium, followed by the ventricle (a thick walled pump). Blood then flows into the conus arteriosus (elasmobranchs) or bulbus arteriosus (teleosts) then to the gills and the rest of the body.

The heart of slow moving fishes is comparatively small, whereas active swimming species such as the Blue Mackerel have large hearts.

Fish Gonads

The gonads are the organs that produce sex cells.

Male fishes usually have paired testes that produce sperm. Females usually have paired ovaries that produce eggs.

The method by which the eggs and sperm meet varies widely among fishes. Many species are broadcast spawners, shedding their eggs and sperm into the water to fertilise external to the body. Others such as sharks and rays have internal fertilisation where the sperm are released into the body of the female.

Fish Kidneys

The kidneys are one of the body organs involved in excretion and regulation of the water balance within the fish.

The kidneys are paired organs located in the body cavity either side of the backbone. Seawater contains more dissolved salts than the body a fish (and freshwater contains less). This means that marine and freshwater fishes have different problems with regard to regulating the concentration of salts within their bodies. Their kidneys differ considerably in structure.

The internal dissection is now completed.

The gills of a fish are usually made up of filaments, arches and rakers. The filaments are the obvious red structures in the image. The arch is the boomerang-shaped structure that supports the filaments. The rakers project from the opposite side of the arch from the filaments.