Pisces or Fishes are the first Gnathostome (jawed) vertebrates and the first successful class of chordates. They are worldwide in distribution. Pisces are aquatic, gill breathing, cold blooded vertebrates.
A fish has a streamlined body so as to facilitate easy movement in water. There are about 26,000 species of fishes and they represent the largest single group of vertebrates.
The smallest fish is Mystichthys lozerenses (1.2cm long) and the largest fish is Rhinodon (20meters long).
Fishes were evolved during the Ordovician period and they dominated during Devonian period. Hence, Devonian period is popularly known as “Golden age of fishes”. Among fishes some became extinct (placoderms) and a few (Coelocanth) remain unchanged from the early ages. So they are called living fossils.
Placoderms were the fore runners of higher fishes. They gave rise to bony fishes first and then to the cartilaginous fishes. The fossil record of bony fishes appeared in the early devonian and that of cartilaginous fishes in mid devonian period.
The branch of science which deals with the study of fishes is called ‘Ichthyology‘.
General Characters Of Fishes :
1. Fishes are aquatic, ectothermal or poikilothermic vertebrates.
2, The body is spindle-shaped or stream lined i.e., the body is laterally
Compressed and pointed at both the ends.
3. The body is covered by various types of dermal scales. They are mesodermal in origin.
4. The body is divided into head, trunk and tail. Neck is absent. It is an adaptation to aquatic mode of life.

5. One pair of external nostrils is present on the anterodorsal surface of the head. They do not communicate with the buccal cavity except in Dipnoi fishes in which internal nostrils are present. In other fishes, they function as olfactory organs.
6. Tympanum and middle ear are absent. Only internal ear is present. It has three semicircular canals.
7. Eye lids are absent in the eyes. The eyes are protected by transparent nictitating membrane.
8. Slime glands are abundantly present in the skin. Their secretion keeps the skin slimy and reduces the friction with surrounding water during locomotion.
9. The locomotion is effected by special appendages called fins. Besides locomotion, they also help in balance. Fins are of two types, paired and unpaired.
10. Pectoral and pelvic fins are paired and help in locomotion. Dorsal, ventral and caudal fins are unpaired in nature and help in balance. The caudal fin functions as a rudder.
11. The endoskeleton may be cartilaginous or bony.
12. A single occipital condyle is present in the skull, and hence the skull is monocondylic.
13. Vertebrae are of amphicoelous typei.e., concavities on both sides of centrum.
14. Respiration is effected by four or five pairs of gill slits. In dipnoi fishes, the air bladder functions as lung.
15. Alimentary canal has definite stomach and well developed liver and pancreas. It ends with anus or cloaca.
16. The heart is’S’ shaped and two chambered with one auricle and one ventricle. Sinus venosus, conus arteriosus or bulbous arteriosus are also present.
17.The heart is venous heart as it contains only impure blood. The circulation is single circulation as the blood flows through heart only once before entering the organs.
18. Red blood corpuscles are oval, biconvex and nucleated.
19. Both hepatic and renal portal systems are present.
20. Brain is small and it does not fill the cranial cavity.
21. Cranial nerves are ten pairs.
22 . Lateral line sense organs are well developed and situated on the head and along the trunk. These organs help to detect the pressure changes in water. So they are called rheo receptors.
23. The functional adult kidney is always a mesonephros. There is no urinary bladder.
24. The nitrogenous excretory waste is in the form of ammonia in bony fishes and urea in elasmobranchs.
25. Sexes are separate (unisexual), gonads are typically paired and the
genital ducts open into cloaca.
26. Most of the fishes are oviparous and a few fishes are viviparous . (Scoliodon)
27. Fertilization is external in oviparous fishes and internal in viviparous fishes.
28. Extra-embryonic membranes are absent.
29. Eggs are large with much yolk. Because the are called as Megalecithal egg.
30. Development is generally direct without any metamorphosis.

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