External features of different kinds of fish

a) Head:

• The region from the snout to the posterior edge of the operculum is the head. The head region consists of the mouth, eyes, operculum, barbells, etc.
• The mouth is terminal in tilapia, sub-terminal in Labeo, and ventral in Schizothora, Garra species,s etc.
• Mouth encircled with sensory barbells in the majority of fishes.
• The large eyes are situated on the lateral side of the head.
• Eyes are very rudimentary in some fishes which live in deep water or muddy water ex. Mastacembellus, Monopterus species.
• On either side of the head region bony structure known as an operculum and beneath each operculum lies respiratory organs.

b) Trunk:

• It is a thick middle part of the body, on either side of the trunk lateral line present or extends from the operculum to the base of the caudal fin which may be continuous discontinuous, or absent.
• The trunk bears fins including paired or unpaired fins.
• The dorsal surface of the body consists of a large dorsal fin supported by fin rays.
• In some fishes, only one unpaired dorsal fin is present while in some fishes second dorsal fin is also present.
• In Some fishes instead of the second dorsal fin, an adipose fin is present without supporting fin rays.
• Just behind the operculum, a pair pectoral fins lies on either side.
• Mid-ventral of the body a pair of pelvic or ventral fins is present in the majority of fishes.
• At the end of the trunk lie series of three apertures, anus and middle genital and posterior urinary.

c) Tail region:

• It is laterally compressed and narrow behind.
• At the ends bearing un paired homocercal caudal fin in majority of fishes.
• While in some fishes caudal fin may heterocercal or diphycercal or protocercal type.
• The single unpaired anal fin lies just behind the anus.

d) Fins:

• Fins act as the organ of locomotion. Fins are made up of rigid rays.
• Fishes that have raysthath are bony, stiff, and separated are called spines.
• In such cases, the front portion of the fish is called Spiny Dorsal, and the rear portion is called Soft Dorsal. In Gobi species, the Spiny and Soft dorsal are separated and form two distinct dorsal fins.
• Dorsal fin and ventral – Helps the fish to be stable while swimming
• Caudal fin – This fin is the main one for propelling.
• Anal fin- Used for stability
• Pectoral fins- Used for propelling and side-to-side movement.
• Adipose fin – Gives Stability while swimming.

e) Skin:

• Fish skin is divided into two layers, the outer layer (Epidermis) and the Dermis.
• The outer layer Epidermis is made up of Epithelial cells. These cells are arranged one above the other and drop frequently and it’s replaced with new ones.
• Slime cells are present in the interspace between the outer cells. The slime coat, a form of mucus is produced by the slime cell and is a very important protective covering.
• UTheEpidermis layer lies under the Dermis layer. This layer is made up of connective Fibroblasts, collagen, and blood vessels.

f) Scales:

• In the pocket of the Dermis lies the scales and it comes out of the connective tissue. The scales are covered by the epithelial layer.
• The scales overlap with each other to form a protective elastic wall that is capable of withstanding blows and bumping.
• Scales are of two types, one is serrated, and the other is smooth.
• In Mailed Catfish bony plates are present instead of scales. Some species do not have scales at all.

g) Gills:

• Gills present under the gill cover help the fish to respire.
• They are the delicate system of blood vessels covered by a very thin Epithelium through which the fish breathes.
• The Pharynx wall is perforated with five sslit-likeopenings. The tissue between the slits is called the Gill arch.
• On each side of the fish present five-gill slits and four-gill arches. The gills are mounted on the gill arches.