Fish are the cold blooded animals which live in water. They breathe in dissolved oxygen through specialised structures called the gills. The fishes are mainly of three types: jawless fish, bony fish, cartilaginous fish.Complete answer:
To answer this question, we must know about the structure of fish.
-The fish is an aquatic animal whose body is modified to live in water. They have a streamlined body which helps it to swim in water.
-The fish-like vertebrates have a notochord, vertebrae and a well defined body with head and tail. The external parts of fish are-
-Scales- The outer part of the fish body is covered with scales which is a part of the integumentary system. It is a part of mesoderm. It is covered with a protective layer or the slime.
-Jaw -The vertebrae jaw of the fish are not only meant for feeding but also helps in increasing the respiration efficiency.
-Eyes- The eye of a fish is similar to mammals and birds. The eyes have a rod and cone for vision in dim light and also colour vision.
-Gills Located under the operculum. It is the respiratory organ for extracting dissolved oxygen from water.
-Lateral line- It is a sense organ which helps in detecting vibrations and movement of water.
-Fins- It is the most important feature of fish which helps it in swimming. The types of fins are anal fin, caudal fin, dorsal fin.Note:
The heart of the fish is two chambered which pumps blood through the gills and the body. The fishes lack an external or middle ear but they have an inner ear. The other internal organs are liver, kidney, spleen, intestine, stomach etc. They also possess reproductive organs and a central nervous system.