We can find mainly two kinds of fishes. They are cartilaginous fishes and bony fishes. Cartilaginous fishes are members of the class Chondrichthyes. In class Osteichthyes, we can find bony fishes. These two kinds of fishes have different kinds of characteristics.
Characteristics of the Class Chondrichthyes
Mainly, in this section we learn about the external, internal morphology and physiology features of cartilage fish. This class contains about 600 species including sharks, skates, rays and ratfishes.
Habits and External Features
These fishes are mostly marine predacious organisms. They have fusiform, streamlined bodies with a head, a trunk and a tail. In most the mouth is ventral in position. Also, there are two well-developed eyes on the head. These are based on the typical vertebrate pattern. Their first gill slit is modified into a spiracle. The anus is placed mid-ventrally at the base of the tail.
Fins and tail shape
We can find both median and lateral fins in cartilaginous fish. Cartilaginous rods of basal and radials and fin rays support these fins. The median fins are unpaired and consist of one or two dorsal fins, a caudal fin and an anal fin.
Skin and scales
Mucus secreted by the mucous gland cells covers the skin. Cartilage fishes characteristically have placoid scales in their skin.
Structure of placoid scales
The structure of the placoid scales resembles that of teeth of higher vertebrates. Mainly, there are two parts in a typical placoid scale. They are the basal plate and a backwardly pointed spine.
Endoskeleton
Chondrichthyes skeleton is fully cartilaginous.
Digestive system
The alimentary canal has a well-developed mouth, a stomach, a short intestine with a spiral valve and an anus. The spiral valve refers to the characteristic fold of the mucous membrane of the intestine. This spiral valve has two functions. They are increasing the absorptive surface of the intestine and preventing the rapid flow of food through the intestine.
Gills and Respiration
In cartilage fishes the gills function in respiration. The gills of cartilage fishes are composed of rows of filaments referred to as the lamellae and thin gill plates. Both lamellae and gill plates increase the total surface area of the gill. These fishes differ from all other fish in having plate-like gills.
Nervous system
The nervous system has two sub systems. The brain and the spinal cord are the main parts of the central nervous system. The peripheral nervous system includes the cranial and spinal nerves and the autonomic nervous system.
Reproduction
In cartilaginous fish, the sexes are separate. Fertilization is internal. The number of eggs produced by cartilage fishes is very small.
Characteristics of the Class Osteichthyes
Bony fishes live in fresh water as well as the sea. Members of this class are known as bony fishes. Because their skeleton is completely bony. This class of fishes, has more than 25,000 species including teleost, lobe finned fishes and the lung fishes.
Body plan and external features
The body of bony fishes is streamlined. Their mouth is usually terminal or sub terminal. Jaws usually bear teeth. External nares are placed posterior to the mouth in a dorsal position. Eyes well-developed but they have no lids. The tail in primitive bony fishes was of the heterocercal type. However, in modern ones, the tail fin is not involved in providing support as it does in cartilage fishes.
Skin and Scales
Bony fish has a skin with many mucous glands. They have three types of scales in their skin.
1. Cosmoid scales:
These scales are not found in any living organisms today, except in the primitive bony fish, latimeria. These scales lie deeper in the dermis and have several distinct layers.
2. Gasnoid scales:
The ganoid scales are rhomboidal in shape. The upper layer of the scale is made up of a hard, highly resistant translucent substance called ganoine. At the base are layers of isopedine.
3. Leptoid scales:
Most teleosts have leptoid scales. It is the ganoid scales that have led to the thinner leptoid scales. There are two types of leptoid scales. They are cycloid and ctenoid scales.
Endoskeleton
The endoskeleton is formed mainly of bones.
Digestive system
The digestive system is similar to that of cartilage fishes consisting of an oesophagus, stomach and intestine. But most bony fishes do not have a spiral valve. Also, these fishes have structures called pyloric caeca between the stomach and intestine. These act as areas of digestion and absorption.
Respiration
Usually four pairs of gills are present in bony fishes. The gills of bony fishes look very different from those of the cartilage fishes. Because the gills of bony fishes are no longer bound together by a septum.
Swim bladder
The tendency to develop a swim bladder as an outgrowth from the ventral pharyngeal region occurred early in the history of bony fishes. This structure has allowed bony fishes to radiate in form and mode of life to occupy as many aquatic habitats as possible.
Locomotion
Bony fishes are usually very active swimmers when compared to cartilage fish. In most other bony fishes, the propulsive force comes from the movements of the tail and the posterior part of the body.
Heart and circulation
The blood vascular system and physiology of circulation are similar to that of the cartilage fishes.
Nervous system
As same as, nervous system of a bony fish is similar to that of a cartilage fish. But, the brain of a bony fish more developed than a cartilage fish.
Reproduction
Sexes are separate. Gonads are paired. There is no cloaca. They do not have claspers either. Fertilization is usually external.