Body structure of fish

Most fishes (except plaice and halibut) have a symmetrical shape, which may be divided into three main parts: head, body and tail. The head is anterior part and extends from the tip of the snout till the end of the gill covers. The body extends from the gill covers to the anal fin and then merges into the tail which is divided into the stem and fin. There are no clear cut boundaries between these parts which pass imperceptibly into one another. The body of fish has several fins: the pectoral or breast fins, the ventral or pelvic fins (paired), and the dorsal and anal fins. The surface of the body and tail-stem is covered with skin, which is the seat of scales or small sharp bony plates. Beneath the skin are the mus­cles which constitute the flesh of the fish supported on a bony or cartilaginous skeleton. The body cavity holds the viscera – a complex of organs fulfilling various physiological func­tions (digestion, reproduction, etc.).

Skeleton. The framework of a fish is its hard internal skeleton consisting of bones or cartilage around which the tissue and organs are grouped. The chief part of the skeleton is the backbone or vertebral column, which consists in a bony fish of a great many vertebrae (56 to 200) joined together by connective tissue into a long, elastic notocord.

Muscles. The musculature of fish includes three groups of striated muscles, those of the head, body, and fins. The bulk of the muscles are the body or stomatic muscles on either side of the vertebral column, and consist of four longi­tudinal, two dorsal and two ventral muscles, separated from one another by strong connective tissue. Located between the muscle fibres and bundles are blood and limphatic vessels and nerves, the space between them being filled with a small quantity of semi-liquid structureless matter (intercellular fluid). The system of blood vessels is particularly dense in the layer of muscles next to the lateral line, which explains why many fishes (tunny, mackerel and others) have dark flesh along the side. The primary muscle fibre or cell, which is the basic morphological and functional element of the body muscles, has a very complicated structure.

Vascular system. The heart of a fish lies in the front part of the abdominal cavity and consists of two parts – the auricle and ventricle. The venous blood in the auricle passes through the ventricle into the ventral aorts and so to the gills, where it absorbs oxygen, and then to the dorsal aorta and head aorta. The largest blood vessels are those deep-seated beneath the vertebral column, and those between the heart and gills. To bleed fish, therefore it is usual to cut the ventral aorts, making the cut between the pecto­ral fins near the branchial arches, in this way the heart con­tinues pulsating and the blood is propelled out of the vas­cular system.

Internal organs. The internal organs or viscera inside the body cavity of a fish are the heart, the digestive organs (oesophagus, stomach, intestine, liver, pancreas), the kidneys, the gonads and the swim bladder. All these organs are freely suspended inside the body cavity by means of loose, sur­rounding connective tissue. The cavity wall is lined with smooth protective tissue, the peritoneum which in some fishes (carp, cod, and others) is covered by an additional thin black membrane, usually removed when the fish are dressed.

The swim bladder has the form of a white, oblong bag, and lies in the upper part of the body cavity between the kid­neys and the alimentary canal. Its chief purpose is to pro­vide boyancy and to help the animal to rise and sink in the water. Many fishes such as flatfish, sharks and skates have no air bladder, i. e. swim bladder. Even such an active swim­mer as the common mackerel has no swim bladder, except when very young.

Exercise 2. Answer the following questions:

1. What parts is the fish body divided into?

2. What fins are there on the body of the fish?

3. What is the surface of the body and tail-stem covered with?

4. What does the body cavity hold?

5. What is the framework of a fish?

6. What groups of muscles does the musculature of fish include?

7. Why do many fishes (tunny, mackerel and others) have dark flesh along the side?

8. What parts does the heart of the fish consist of?

9. What do the internal organs or viscera inside the body cavity include?

10. What is chief purpose of swim bladder?