Rabbit (Oryctolagus cuniculus)

Rabbit (Oryctolagus cuniculus)

Taxonomic Position

Phylum : Chordata

Sub-phylum : Vertebrata

Class : Mammalia

Order : Lagomorpha

Genus : Oryctolagus

Species : cuniculus

1. Habit and Habitat

Rabbits are gentle and timid animals. They show leaping movement and live in burrows.

They are distributed throughout the world. They are herbivorous animals feeding on grass and vegetables like turnips, carrots and lettuce. Rabbits are gregarious (moving in groups) animals

2. External Morphology

Shape, Size and Colouration: It has an elongated and cylindrical body. Males and females are of the same size. They grow about 45 cm in length and weigh about 2.25 kg as adult. The colour varies from white to black and white. Body is covered with fur which serves to keep it warm.

Body-division: The body of the rabbit is divisible into the head, neck, trunk and tail.

Head: Head is ovoid, flattened and bears a truncate snout. It contains mouth, external nares, eyes, ears and vibrissae. The mouth is a transverse slit-like bounded by upper lip and lower lip. Just above the mouth are two oblique openings called nostrils. From each side of the upper lip tactile hairs or vibrissae (whiskers) project outwards. A pair of large, movable external ear or pinnae is situated at the top of the head.

Neck: The neck connects the head with the trunk. It helps to turn the head.

Trunk: The trunk is divisible into an anterior thorax and a posterior abdomen. In females, four or five teats or nipples are present on the ventral surface between the thorax and abdomen.

The trunk bears two pairs of pentadactyl limbs. The forelimbs are shorter than the hind limbs. All the digits bear claws.

The anus is present at the posterior end of the abdomen at the base of tail. In females on the ventral side a slit like vulva is present. In males penis is present in the ventral side of anus. The male has a pair of testes enclosed by scrotal sacs.

Tail: The tail is short. It is used to give signals to other rabbits in the event of danger.

Integument (Skin): The integument forms the outer covering of the body. The structures which are derived from it are hairs, claws, nails and glands like sweat glands, sebaceous glands and mammary glands.

Mammary glands are modified glands of the skin. They secrete milk and help in nourishing young ones. The sweat glands and sebaceous glands embedded in the skin regulate the body temperature.

3. Coelom (Body cavity)

Rabbit is a coelomate animal. The body is divisible into thoracic cavity and abdominal cavity separated by transverse partition called diaphragm. Diaphragm is the characteristic feature of mammals. Breathing movements are brought by the movement of the diaphragm.

Lungs and heart lie in the thoracic cavity, whereas, abdominal cavity encloses digestive and urinogenital system.

4. Digestive System

The digestive system includes the alimentary canal and the associated digestive glands . The alimentary canal consists of mouth, buccal cavity, pharynx, oesophagus, stomach, small intestine, caecum, large intestine and anus.

Mouth is a transverse slit bounded by upper and lower lips. It leads into the buccal cavity. The floor of the buccal cavity is occupied by a muscular tongue. Jaws bear teeth.

The buccal cavity leads into the oesophagus through the pharynx. Oesophagus opens into the stomach followed by small intestine. Caecum is a thin walled sac present at the junction of small intestine and large intestine. It contains bacteria that helps in digestion of cellulose. The small intestine opens into the large intestine which has colon and rectum. The rectum finally opens outside by the anus.

Digestive glands

The digestive glands are salivary glands, gastric glands, liver, pancreas and intestinal glands. The secretions of digestive glands help in digestion of food in the alimentary canal.

Dentition in Rabbit

Teeth are hard bone- like structures used to cut, tear and grind the food materials. The

rabbit has two sets of teeth. The existence of two sets of teeth in the life of an animal is called diphyodont dentition. The two types of teeth are milk teeth (young ones) and permanent teeth (in adults).

In rabbit the teeth are of different types. Hence, the dentition is called heterodont. There are four kinds of teeth in mammals viz. the incisors (I), canines (C), premolars (PM) and molars (M). This is expressed in the form of a dental formula.

Dental formula is the simple method of representing the teeth of a mammal. The number of each kind of tooth in the upper and the lower jaws on one side is counted.

Dental formula is (I  2/1,  C 0/0, PM. 3/2, M 3/3.) in rabbit which is written as 2033/1023. Canines are absent. The gap between the incisors and premolar is called diastema. It helps in mastication and chewing of food in herbivorous animals.

5. Respiratory System

Respiration takes place by a pair of lungs, which are light spongy tissues enclosed in the thoracic cavity. The thoracic cavity is bound dorsally by the vertebral column and ventrally by the sternum, laterally by the ribs. On the lower side of the thoracic cavity is the dome shaped diaphragm.

Each lung is enclosed by a double membranous pleura . Atmospheric air passes through the external nostril and nasal passages into the pharynx. From the pharynx it passes through the glottis into the wind pipe.

The anterior part of the wind pipe is enlarged to form the larynx or voice box with its wall supported by four cartilaginous plates. Inside the larynx lies the vocal cord and its vibrations result in the production of sound. The larynx leads into trachea or wind pipe.

Tracheal walls are supported by rings of cartilage which help in the free passage of air. The epiglottis prevents the entry of food into the trachea through the glottis. The trachea divides into two branches called the bronchi one entering into each lung and dividing into further branches called bronchioles which end in alveoli.

The respiratory events consist of inspiration (breathing in) and expiration (breathing out) allowing exchange of gases (oxygen and carbon dioxide). Inspiration is an active process while expiration is a passive process.

6. Circulatory System

The circulatory system is formed of blood, blood vessels and heart. The heart is pear shaped and lies in the thoracic cavity in between the lungs. It is enclosed by pericardium, a double layered membrane.

The heart is four chambered with two auricles and two ventricles. The right and left auricles are separated by interauricular septum, similarly right and left ventricles are separated by interventricular septum.

The right auricle opens into the right ventricle by right auriculoventricular aperture, guarded by a tricuspid valve. The left auricle opens into the left ventricle by left auriculoventricular aperture guarded by a bicuspid valve or mitral valve. The opening of the pulmonary artery and aorta are guarded by three semilunar valves.

The right auricle receives deoxygenated blood through two precaval (superior vena cava) and one postcaval (inferior vena cava) veins from all parts of the body. The left auricle receives oxygenated blood from the pulmonary veins from the lungs. From the right ventricle arises pulmonary trunk which carries the deoxygenated blood to the lungs and from the left ventricle arises the systemic arch (aorta) which supplies oxygenated blood to all parts of the body.

7. Nervous System

The nervous system in rabbit is formed of the central nervous system (CNS), peripheral nervous system (PNS) and autonomic nervous system (ANS).

CNS consists of brain and spinal cord. PNS is formed of 12 pairs of cranial nerves and 37 pairs of spinal nerves. ANS comprises sympathetic and parasympathetic nerves.

Brain is situated in the cranial cavity and covered by three membranes called an outer duramater, an inner piamater and a middle arachnoid membrane. The brain is divided into forebrain (prosencephalon), midbrain (mesencephalon) and hindbrain (rhombencephalon).

Forebrain consists of a pair of olfactory lobes, cerebral hemispheres and diencephalon. The right and left cerebral hemispheres are connected by transverse band of nerve tissue called corpus callosum.

The midbrain includes the optic lobes. The hindbrain consists of the cerebellum, pons varolii and medulla oblongata. You will study about the details of mammalian brain in the chapter on nervous system.

8. Urinogenital System

It comprises the urinary or excretory system and the genital or reproductive system. Therefore, they are usually described as urinogenital system in vertebrates.

Excretory system

Each kidney is made of several nephrons. It separates the nitrogenous wastes from blood and excretes it in the form of urea. Kidneys are dark red, bean shaped organs situated in the abdominal cavity. From each kidney arises the ureters which open posteriorly into the urinary bladder and leads into a thick walled muscular duct called urethra.

Reproductive System

Sexual dimorphism is exhibited in rabbits. The male and female sexes are separate and are morphologically different.

Male Reproductive system

The male reproductive system of rabbit consists of a pair of testes which are ovoid in shape. Testes are enclosed by scrotal sacs in the abdominal cavity. Each testis consists of numerous fine tubules called seminiferous tubules. This network of tubules lead into a coiled tubule called epididymis, which lead into the sperm duct called vas deferens. The vas deferens join in the urethra just below the urinary bladder. The urethra runs backward and passes into the penis.

There are three accessory glands namely prostate gland, cowper’s gland and perineal gland. Their secretions are involved in reproduction.

Female reproductive system

The female reproductive system of rabbit consists of a pair of ovaries which are small ovoid structures. They are located behind the kidneys in the abdominal cavity.

A pair of oviducts opens into the body cavity by a funnel shaped opening from each side of the ovary. The anterior part of the oviduct is the fallopian tube. It leads into a wider tube called the uterus. The uterus join together to form a median tube called vagina. The common tube is formed by the union of urinary bladder and the vagina and is called the urinogenital canal or vestibule. It runs backwards and opens to the exterior by a slit-like aperture called vulva.

A pair of Cowper’s gland and perineal gland are the accessory glands present in the female reproductive system.