Rat Classification and Body Structure

RAT (Rattus norvegicus)

Phylum - Chordata Rattus rattus - Black rat

Sub phylum - Vertebrata / Craniata Mus musculus - House mouse

Super class - Gnathostomata Rattus albinus - White rat

Class - Mammalia

Order - Rodentia

Genus - Rattus

Species - norvegicus

Salient Features

Terrestrial mammals. Live in groups and are burrowing in nature.

Nocturnal but are active by day time also.

Has low, cylindrical, elongated body well adapted for its burrowing habit. Body has bilateral symmetry.

The colour of the rat may also vary. It can be black rat or brown rat or pure white also. The body of rat is covered by closely arranged short hair coat called pelage. It provides colour,

conserves body heat and protects skin. Hair are absent at the end of the snout, ear-tip, palms, soles and are less on the upper surface of hands and feet and on the tail.

External Features

• Body divisible into 4 parts: head, neck, trunk and tail.
• Head elongated, tapering into a naked snout or muzzle. It is very sensitive to touch and is short, blunt in brown rat. The head bears mouth, eyes, external nares and ears.
• On both sides of nostrils, the bristles are developed as specialised tactile hair and are known as pili tactiles or vibrissae.
• Mouth is small, sub-terminal, opens into buccal cavity. There is a partition in the middle of upper lip and is called hare-cleft or hare lip.
• Eyes lie on the sides of the head. Each eye is provided with upper and lower eyelids bearing fine short hair, the eyelashes, third eyelid or nictitating membrane is present in the anterior corner of the eye. It is supported by a semilunar cartilage. Nictitating membrane covers the eye when head is under water.
• Secretion of lacrymal or tear glands keep the eyes moist. Meibomian glands are present at the base of eyelashes and form an oily cover above it.
• A pair of external ear are located on the postero-lateral parts of the head. It consists of a short, oval pinna enclosing external auditory canal (meatus). The tympanum and eardrum is present at the inner end of the meatus.
• A pair of external nares are situated at the tip of the snout. These open into nasal passage and can be closed under water.
• Long, stiff, bristle-like hair are present around mouth, external nares and above the eyes. These are very sensitive to touch and help the animal to judge the width of the burrow.
• Neck is the link between head and thorax.
• The trunk consists of two parts: thorax and abdomen. The female bears six pairs of nipples or mammae or teats: three pairs on the thorax and three pairs on the abdomen. The trunk bears two pair of limbs, anus and external sex organ.
• Forelimbs are smaller than the hindlimbs and consist of 3 parts:
• (i) The proximal upper arm or brachium - directed backward from the shoulder.
• (ii) The middle forearm or antebrachium - directed forward from the elbow.
• (iii) Distal hand or manus directed foreward.
• The manus consists of the wrist or carpus, palm or metacarpus and 5 fingers or digits. The first digit is thumb or pollex and is reduced. Except the first digit, all the digits end in horny, pointed claws.
• The hand has 5 apical pads at the tips of the digits, 3 interdigital pads on the palm and 2 basal pads at the base of the palm. (Total 10 walking pads).
• The hindlimb consists of proximal thigh femur, middle shank or crus and distal foot or pes. Thigh and pes are directed forward while crus is directed backward.
• The pes consists of tarsus, metatarsus and 5 toes or digits. All the toes bear pointed claws. The foot has 5 apical pads at the tip of the digits, 4 interdigital pads on the sole and 2 basal pads at, the base of the sole.
• At the base of the tail, anus is present i.e. outlet of alimentary canal.
• In female rat, vulva is present in front of anus and functions as genital aperture. It is closed by a plug till puberty. The urethral orifice is in front of the vulva and is for elimination of urine.
• A pair of prominent, thin-walled sacs hang from the abdomen in male rat. These are scrotal sacs which are not clearly separated externally but internally division is complete.
• Testes descend into these sacs when the male becomes one month old. A small cylindrical organ, penis, is present in front of the scrotal sac. It is the male copulatory organ and has at its tip urinogenital aperture; common aperture for urine as well as sperms.
• Perineum is the area between anus and scrotal sac in case of male or anus and vulva in case of female.
• Tail is cylindrical, tapering and as long as the rest of the body. It is covered with little overlapping scales. Tail hair are reduced to 2-3 short bristle which project from under the edge of each scale.
• Body cavity (coelom) is divided into two compartments by diaphragm: anterior thoracic cavity and posterior abdominal cavity.
• Thoracic cavity includes two large pleural cavities for lungs and a small pericardial cavity for heart.
• Skull has two occipital condyles (dicondylic). Teeth are heterodont, diphyodont and thecodont present with both the jaws. Dental formula .

The coelom is the body cavity within which the viscera (internal organs) are suspended. The abdominal cavity and viscera are covered by a membranous tissue called the peritoneum, which is formed from mesoderm. The peritoneum is extensive and forms the following delicate membranes.

(1) Mesentery Proper — A double layer of peritoneal membrane extending from the dorsal body wall to the viscera.

(2) Falciform Ligament — A layer of tissue extending from the ventral body wall and diaphragm, to the  liver.

(3) Parietal Peritoneum — Membrane which lines the body wall of the abdominal cavity.

(4) Greater Omentum — A double walled peritoneal sac which extends from the greater curvature of the stomach over the intestine.

(5) Lesser Omentum — This membrane joins the lesser curvature of the stomach to the liver.