Fish: Structure and Function Study Guide


Fishes are marine animals that have a streamlined body that helps them swim without much resistance in the water, and they also have fins to assist them in swimming.

General characteristics of Fish

  • They are bilaterally symmetrical organisms, and they are triploblastic organisms.
  • Fishes have segmented bodies. Internal segmentation is present.
  • The majority of the fish are spindle-shaped, with others being Dorso-ventrally depressed, laterally compressed, snake-like (Mastacembelus), and globe-like (Tetradon).

Fish Anatomy

Fish anatomySource


  • Scales and bony plates cover the fish’s body.
  • Scales are recognized as a fish’s identity card because of their many roles.
  • The genesis of scales is mesodermal.
  • Siluriformes fishes have no scales (catfishes), and few fishes (major carps) have scales on their heads.


  • Fins help in swimming and balance.
  • Fin rays support the fins, and fins feature both spiky and soft rays.
  • Fins without fin rays are known as adipose fins (Mystus).
  • Fins are divided into two categories: paired and unpaired fins.
  • Pectoral and pelvic or ventral fins are paired fins.
  • The dorsal, anal, and caudal fins are the only ones that aren’t paired.
  • Fins are either typical or modified in the majority of fish.


  • During swimming, the tail is important for shifting direction.
  • The caudal fin is the last fin on the tail.
  • The caudal fin can be forked, circular, or confluent with the dorsal and anal fins.

Closed circulatory system:

  • Fishes have a closed circulatory system.
  • The heart is divided into two chambers, is venous and tubular, and has either a conus or bulbous arteriosus and RBCs have a biconvex shape.

Nervous system:

  • Cerebrum is not well developed in the nervous system, and sharks, in particular, have well-developed olfactory lobes.
  • There are ten pairs of cranial nerves discovered.


  • Branchial respiration occurs via gills.
  • The branchial chamber is where the gills are found.
  • There are 5-7 gills in cartilaginous fishes, but in teleosts, there are 3- 5 gills.
  • Gill arch supports each gill, gill lamellae facilitate gaseous exchange, and gill rakers are extensively developed in plankton-feeding fishes, where they serve as a filter.
  • In cartilaginous fishes, the branchial system is open, but the branchial system is closed in bony fishes.
  • The operculum is exclusively seen in bony fish.
  • There are many pairs of external branchial apertures, but there is only one pair in bony fishes.
  • Hemoglobin is a pigment found in the blood.


  • Mesonephric kidneys are found in ammnotelic animals’ excretory systems.
  • Marine fish keep urea in their blood to stay in an isotonic state with saltwater.

Economic Importance:

  • They’re eaten worldwide, and the fish liver is the primary source of vitamin A and D-rich liver oil.
  • Fish oils are utilized externally in the soap industry and tanneries.
  • Beautifully colored fishes are currently the vogue in aquariums.


  • Fishes are water animals having a streamlined body that helps them swim without much resistance in the water, and they also have fins to assist them in swimming.
  • They are bilaterally symmetrical organisms.
  • They are triploblastic organisms.
  • Scales and bony plates cover the fish’s body.


1. What is the structure of a fish?

Fish are cold-blooded creatures having a backbone, gills, and fins. The head, trunk, and tail make up a normal fish’s body. Two eyes, each with a well-developed nictitating membrane, two internal ears, two nostrils closed internally (except in lungfishes), and a mouth are found on the head.

2. What is the function of the fish organ system?

Gills help fish to “breathe” oxygen from the water. Water enters through the mouth, travels through the gills, and leaves through a specific aperture in the body. As the water travels across them, the gills absorb oxygen.

3. What is the body structure for the adaptation of fish?

They have gills that allow them to breathe in water without difficulty, a streamlined body that allows them to swim easily in water, cold-blooded, which allows them to survive in cold water, and general fins that aid in swimming.

4. What is the structure of fins in fish?

Fins are anatomical elements that protrude from a fish’s body and are made up of bony spines or rays. They are covered in skin and are attached in a webbed pattern, as seen in most bony fish, or a flipper-like pattern, as seen in sharks.

5. Do fish sleep?

While fish do not sleep in the same manner as humans do, most fish do rest. According to research, fish may limit their activity and metabolism while remaining attentive to danger. Some fish float in place, while others find a haven in the mud or coral, yet others find a suitable nest.

6. Do fish poop?

Yes, fishes do excrete the food they ingest. Fish use their gills and skin to pee and poop. Some also pee and defecate through a tiny hole at the back of the body called a pore. Fish take a long time to defecate because their digestive systems become clogged with the food they eat from time to time.

8. Do fish gets thirsty?

The answer is no; they live in water, so they are unlikely to seek out and consume water as a deliberate response. Thirst is commonly characterized as a strong urge or need to consume water, and such a driving force is unlikely to elicit a response from fish.

9. How do fish see at night?

Deep-sea fish have figured the how to navigate in full darkness. Fish have rows of pressure-sensitive organs running down either side of their bodies called the lateral line that allow them to feel surrounding creatures based on pressure fluctuations in the water.

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  1. Fish Structure and Function Accessed on 3 Dec 2021
  2. Structure and Function – Fish accessed on 3 Dec 2021
  3. Fish anatomy Accessed on 3 Dec 2021
  4. Fish Accessed on 3 Dec 2021

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