Pisces are aquatic animals that breathe through their gills, which are permanently attached to their bodies. They are devoid of a genuine spine in the traditional sense. Fish can be found in all types of water, including fresh, marine, and brackish. Fins are the organs that fishes use to propel themselves around. Fins, on the other hand, are sometimes paired or unpaired. Ichthyology is the scientific study of fishes. It is believed that they developed from sea-squirt, which is similar to a primitive fish at this point.

Characteristics

• The following are the physical characteristics of this class: 
• They have a streamlined body 
• Their circulatory system is of the close type.
• The body is divided into three sections: the head, the trunk, and the tail.
• The skeleton might be made of bone or cartilaginous material.
• There are separate facilities for men and women.
• There is evidence of a well-developed digestive system.
• There are no extraembryonic membranes present.
• Fertilization, can be internal or external.
• The body is made up of scales, which serve as an external covering for the creature.
• They can be herbivorous, carnivorous, or omnivorous depending on their diet.

Classification of Pisces

They have further subdivided the group into three categories:

Placodermi

This group of fishes comprises species that are no longer alive. They are made up of fish that lived millions of years ago. The fossil record provides the only way to learn about these fishes. Placoderms are the world’s earliest fishes to have teeth.

1. Placoderms have the following characteristics:

•  They were able to persist until the Permian epoch of the Palaeozoic era.
• The endoskeleton is made of bone.
• Placodermi had a heterocercal caudal fin, which was unusual.

1. Chondrichthyes

• Chondrichthyes are also referred to as elasmobranchs in some circles. The following are the features of the class Chondrichthyes: 
• The absence of an operculum (gill covering).
• They can only be found in maritime environments.
• The skin is made up of placoid scales that are smooth and shiny. Placoid scales are also referred to as denticles in some cases.
• The caudal fin has a heterocercal structure. Sharks have heterocercal tails, which are different from one another.
• These fishes do not have an air bladder in any way shape or form.
• There is an ampulla of Lorenzini present.
• They have a jaw suspension that is either amphystylic or hyostylic. Amphistylic jaw suspension is characterised by the fact that the upper jaw is braced against the cranium and is supported by the hyomandibular joint. As a result, this type of jaw suspension is found in some shark species.
• Males are endowed with a pair of claspers. These claspers serve as the reproductive portion of the organism.

1. Osteichthyes

These are referred to as “bony fishes” in some circles. Furthermore, this is the most numerous class of vertebrates. Furthermore, it contains approximately 29,000 different species. They are endowed with bony internal skeletons.

Osteichthyes have the following characteristics:

• They can be found in seawater, freshwater, or brackish water. 
• They lack the claspers that are necessary for male reproduction.
• The jaw suspension of Osteichthyes is autostylic, which means it moves independently of the rest of the body. It is common for this type of jaw to have the upper jaw directly attached to the brain. 
• The air bladder is frequently present.
• In addition, the operculum, which is the covering of the gills, is present in this class.
• The Osteichthyes are divided into three subclasses: the first is the Dipnoi, the second is the Crossopterygii, and the third is the Actinopterygii.
• Dipnoi is made up of lungfishes, such as the South American lungfish, among others.
• Crossopterygii have fins that are arranged in pairs and are used to provide support in the water.

Respiration

Fishes breathe through their gills, which allow them to stay alive. On either side of the pharynx, some gills are present. This is where the gaseous exchange takes place. The process of taking in oxygen and exhaling carbon dioxide. For example, sharks and lampreys have a large number of gill openings. Bony fishes, on the other hand, have only one aperture in their mouth.

Excretion

Urea excretion is the most significant excretion in fishes. As a result, they are capable of excreting ureotelic excrement. Gills, on the other hand, can function as excretory organs as well. Freshwater fish excrete significantly more water than saltwater fish. Unique organs such as kidneys may be present in some of them.

Circulation

The flow of blood in fish is only one way, and it only goes in one direction. It has a circulatory system that is closed. Also notable is the fact that it only has two chambers in its heart. The heart is divided into four different chambers: the sinus venosus, the atrium, the ventricle, and bulbous arteriosis (aortic arch). The heart pumps blood to the gills, which are located in the kidneys. Aside from that, the blood is oxygenated in the gills before being distributed throughout the body.

Digestive system

Food is taken in through the mouth during digestion. The oesophagus is where food is broken down, while the stomach is where digestion takes place. Pyloric caeca are found in the stomachs of several fishes. Secretion of digestive enzymes is accomplished by the use of finger-like pouches. It is also responsible for the absorption of nutrients. The intestine is present to aid in the absorption of nutrients from the stomach.

Central Nervous System

Both the central nervous system and the peripheral nervous system can be split. First and foremost, there is the central nervous system (CNS), and then there is the peripheral nervous system (PNS) (PNS). The central nervous system is made up of the brain and the spinal cord, among other things. Furthermore, the nervous system is a descendant of the ectoderm tissue.

Organs of Sensation

The sense organs associated with Pisces are as follows:

1. Eyes

Fishes have eyes that are specially designed to see in the water. Furthermore, their eyes are completely devoid of lids. The eye is made up of several layers. They are equipped with a spherical lens. Furthermore, certain fishes are capable of seeing ultraviolet (UV) or polarised light spectrums.

2. Hearing 

They have ears and lateral lines for hearing, which is a good thing.

The reproductive system 

The sexual organs are located within the body. Female fish are made up of ovaries and testicles from male fish. Furthermore, the female fish produces eggs, while the male fish produces sperms. Fertilization occurs when the eggs and sperms combine.

Conclusion

However, in the majority of cases, they do not engage in direct mating. For their part, the female fish spreads her eggs in the water while the male fish exhales its sperm into the water. As a result, external fertilisation is carried out. Furthermore, in fishes that have internal fertilisation, the eggs either hatch internally or are discharged before hatching, depending on the species.