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Pisces (Fish): An Introduction

MVSAT 2024

Pisces are aquatic creatures with gills that allow them to breathe. They don't have a genuine spine. Fresh, marine and brackish water may all contain fish. Fins are the motility organs in fish. Fins, on the other hand, can be paired or unpaired. Ichthyology is the study of fishes. Meanwhile, they are thought to have originated from sea squirts, which are similar to primitive fish.

Pisces Description

Pisces are true jawed vertebrates with specialised respiratory, excretory, and circulatory systems. Since the species in this class are poikilotherms, they are unable to control their own body temperatures. In essence, this category includes all fish.

They have breathing and motility organs that are associated with permanently aquatic life. The gills are the respiration organs, while the paired and damaged fins are the movement organs. They're all poikilothermous. In the phylum Pisces, catfish, goldfish, whales, sharks, and jellyfish are a few examples. Various interesting Pisces facts are known throughout the world.

General Characteristics of Pisces

The following are the distinct Pisces traits:

  • Aquatic vertebrates, either freshwater or marine, herbivorous or carnivorous, cold-blooded, oviparous, or ovoviviparous.

  • The body is normally streamlined and spindle-shaped, with some elongated snake-like features and a few dorsoventrally compressed features, and is divided into three parts: head, trunk, and tail.

  • True dermal fin-rays assist locomotion by paired pectoral and pelvic fins, as well as median dorsal and caudal fins. Propulsion is accomplished through the use of a muscular tail.

  • An exoskeleton of dermal scales, denticles, or bony plates covers the body surface, which is a striking characteristic of Pisces.

  • The endoskeleton is made up of cartilaginous or bone tissue. Vertebrae, either bone or cartilage, frequently replace the notochord. The presence of a well-developed skull and a system of visceral arches, the first pair of which forms the upper and lower jaws with the latter articulated with the skull.

  • Muscles are divided into myotomes, which have separate dorsal and ventral sections.

  • An alimentary canal with a distinct stomach and pancreas leads to the cloaca or anus.

  • Gills are the organs of breathing.

  • The heart is two-chambered and venous, with one auricle and one ventricle.

  • The mesonephros of the kidneys is number ten.

  • There are five sections in the brain. There are ten pairs of cranial nerves.

  • There is no tympanic cavity or ear ossicles. There are three semicircular canals in the internal ear.

  • There is a well-developed lateral line system.

  • The sexes are separated. Gonads are usually found in pairs. Gonoducts can either enter into the cloaca or open independently.

  • Internal or external fertilisation is present.

Types of Scales

Placoid Scales Diagram

Placoid Scales Diagram

Fish scales serve as the creature's exoskeleton, which makes sense considering that the word "exoskeleton" refers to a skeleton that is visible from the outside. Fish scales come in a variety of shapes and sizes, including placoid, ganoid, cycloid, cosmid, and ctenoid. Scales are thin, cornified structures that are tiny and thin.

  • Since cosmid scales were a distinguishing feature of many extinct fish such as ostracoderms and placoderms, they are no longer present (or only very rarely found) on fish in the wild today.

  • Placoid scales are the characteristic feature of cartilaginous fishes.

  • Ganoid scales resemble little rhomboids. On the skin of a fish, these diamond-shaped plates fit side by side. Ganoid scales are a distinctive feature of chondrosteans, a group of fish having a cartilage-based skeleton that includes sturgeon.

  • In addition to being thin and translucent, cycloid scales have a somewhat round look. Bone makes up the thin upper layer of these scales, while connective tissue makes up the lower layer. Bowfin, cod, carp, and some lungfish have cycloid scales on their bodies.

  • Ctenidium is the name for anything that has a comb-like look or design. Ctenoid scales feature comb-like projections on the rear as a result. The bodies of flounder and sunfish are covered in ctenoid scales.

Classification of Fishes/Pisces

The classification of fishes pdf contains three main classes which are described as follows:


  • It contains fish that are no longer alive.

  • All of them are armoured fish. Bony plates or shields serve as their exoskeletons.

  • They have a bony endoskeleton.

  • These fishes have an autodiastylic jaw suspension.

  • The caudal fin of a heterocercal is visible.

  • The jaws will not be supported by the hyoid arch.

  • There are primitive jaws visible.

  • They lasted until the Permian epoch of the Paleozoic era.

  • Both Bothriolepis and Climatius are examples of this class.

Elasmobranchi or Chondrichthyes

  • These fish are only found in the sea.

  • Placoid scales make up the exoskeleton.

  • Cartilage is the name for their cartilaginous endoskeleton.

  • Amphistylic or hyostylic jaw suspension.

  • There are 5-7 pairs of gills present.

  • The external gill holes are distinct from the internal gill openings. Operculum does not cover them.

  • There is a heterocercal caudal fin present.

  • Males display claspers in preparation for copulation.

  • These fishes lack an air bladder.

  • Example: Scoliodon and Pristis.


  • These fish can be found in the sea, freshwater, and brackish water.

  • The exoskeleton will be made up of cycloid, ctenoid, or ganoid scales.

  • Bony endoskeleton.

  • Jaws suspension is self-adjusting.

  • There is tuberculosis present.

  • Claspers are not present.

  • In most cases, an air bladder is present.

  • Example- Salmon and Neoceratodus.


The Pisces phylum includes the first jawed vertebrates, and the study of fish is called Ichthyology. They are aquatic (may be freshwater or marine). The traits of Pisces include the presence of a streamlined body covered with different types of dermal scales like Placoid, Cosmoid, Ganoid, Cycloid, and Ctenoid. Fish are cold-blooded animals. This simple division of superclass Pisces into three classes- Placodermi, Chondrichthyes, and Osteichthyes.

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FAQs on Pisces

1. How do fish respire?

Gills are the organs of fish that are responsible for respiration. Both the left and right sides of the pharynx include gill slits. This is where the interchange of gaseous substances takes place. Oxygen is consumed while carbon dioxide is exhaled in this process. Examples of animals with several gill holes include sharks and lampreys. Bony fish, on the other hand, have only a single aperture in their bodies. In addition to that, there are a few species of fish that are equipped with lungs for breathing air, like West African lungfish, marbled lungfish, and spotted lungfish.

2. Describe the nervous system of the phylum Pisces.

There are two sections to the nervous system. The central nervous system (CNS) comes first, followed by the peripheral nervous system (PNS). The brain and spinal cord make up the central nervous system. The nervous system is also derived from ectoderm. The peripheral nervous system also connects nerves that connect muscles and sensory organs to the central nervous system. During swimming, the nervous system is responsible for control and coordination. There are ten cranial nerves in some fish, such as Scoliodon.

3. How does circulation occur in fishes?

Blood can only circulate in one direction through the body of a fish. Its vascular system is completely sealed up. In addition to that, there are only two chambers in the heart that make up this organ. Four separate chambers make up the heart: the bulbous arteriosus, the atrium, the ventricle, and the sinus venosus. The heart acts as a pump, bringing blood to the gills from the rest of the body. In addition, the gills are responsible for oxygenating the blood before it is distributed throughout the body.

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