The class Pisces of phylum chordate includes fishes. Fishes are aquatic animals that lack limbs with digits. They are cold blooded and have two chambered heart. Body of fishes may be long, laterally compressed and spindle shaped. They are egg lying and their fertilization is external. They consist of head, trunk and tail. Fishes are important source of food worldwide. There are about more than 32,000 different species of fish worldwide. Fishes range in adult length from less than 10 mm to more than 20 meters and in weight from about 1.5 grams to many thousands of kilograms. Some live in shallow thermal springs at temperatures slightly above 42 °C, others in cold Arctic seas a few degrees below 0 °C or in cold deep waters.
Classification of fishes
Fishes are broadly classified into two subclasses.
- Super class agnatha
- Class Ostracodermata (Extinct / fossil)
- Class Petromyzontia (Lampry)
- Class Maxinoidea (Hag fish)
- Super class gnathostoma
- Class Placodermi – First Jawed Fish (Fossils)
- Class Chondricthyes – Cartilaginous Fish
Subclass Elasmobranchii – Shark, Rays
Subclass Holocephali – Chimaera, rat fish
- Class Osteicthyes – Bony Fish
Subclass Actinopterygii – Ray-finned Fish; Goldfish; Sea Horse
Subclass Sarcopterygii – Lobe-finned Fish; Coelocanth
Skeleton Of Class Ostracodermata
Ostracoderm have both endo and exoskeleton. Exoskeleton consist of bony armour which protect the body. Cranial region consist of larger dermal bones forming head shield but posteriorly along the body the dermal bone break up into smaller pieces called dermal scales. Scale’s surface is often ornamented with tiny mushroom shaped tubercle. A layer of enamel or enamel like substance is present over the inner layer dentine in the tubercle. Pulp cavities may be one or more residing within each tubercle. These tubercles are supported by dermal bone called lamellar which is organized in layers. They are among the earliest creatures which have bony head. The dermal bone that covered the head is a tooth like structures that fused to make head encasing.
Endoskeleton is not highly ossified. It is consist of fibrous connective tissue, bone and cartilaginous elements. Ostracoderm have no vertebrae rather they have notochord. Head consist of endoskeleton of endochondral bone and considerable cartilage. Their endoskeleton is not highly developed, usually of cartilage. They have no jaws. They have heterocercal tail.
Skeleton of Class myxiniodae
They have cartilaginous endoskeleton. Adult and larva had notochord. They have slime gland which secrete slime on the surface of body. This slime protect the fishes from other animal which attack on them. It also help in swimming. Their body is scale less. They have cartilaginous skull. The part surrounding the head is composed of fibrous sheath. They have tooth like structure made of keratin. They have notochord not vertebrae.
Skeleton of Class pteromyzontia
They have cartilaginous endoskeleton. They have cartilaginous patches of vertebrae along the notochord. They lack a mineralized skeleton but they have cartilaginous neurocranial and viscerocranial elements. The visceral skeleton develops as a fused branchial basket supporting the pharynx. The skeletal rods develop from condensations of flattened discoidal chondrocytes and may involve cellular intercalation. Lamprey trabecular, parachordal, and subchordal cartilages consist of aggregations of polygonal chondrocytes positioned on the ventral and lateral surfaces of the notochord.
Skeleton of Gnathostomes
Skeleton of placoderms
They evolved from Agatha (jawless fishes). They are first that have jaws. They are characterized by heavy armored bony plates. A heavy armor dermal shield cover across head and thorax regions. The rest of body is covered with smaller scales or naked. Cranial shield (dermal plates around head) is thick. Upper jaws are attached to braincase (highly ossified). They have unusual joint in the dorsal armor between the head and neck regions. This joint apparently allowed the head to move upwards as the jaw dropped downwards.
Skeleton of chondyrictheis
They have cartilaginous skeleton and lack true bones. Only their teeth and sometime vertebrae are calcified but this calcified cartilage very different from true bones. During development, notochord is gradually replaced by a vertebral column except in Holocephali, where the notochord remain intact. The column is reduced in some deep water sharks. They are covered with dermal teeth also called placoid scales or dermal denticles which feel like sandpaper. Their oral teeth evolved from dermal denticles which migrated into the mouth.
Skeleton of Oesticthyise
They have bony skeleton. Their exoskeleton is made of thin bony plates. On the basis of their appearance, bony fishes have several types of scales. They have smooth and overlapping ganoid, cycloid or ctenoid scales. Mucus are at help in fighting off bacterial and fungal infection at the surface. The placoid scales are absent in them. They have vertebral column, cranium, ribs, skull and intramuscular bones. The head and pectoral girdles are covered with large dermal bones. The braincase or neurocranium, is frequently divided into anterior and posterior sections divided by a fissure. Bony fish also have an operculum (a bony flap of skin over their gills that protects the gills). Skeleton of fishes give structure, provide protection and is also a site of red blood cell production.