Mammalia (Mammals) - Rat
The body is divided into a head, neck, trunk and tail and is covered by hair. The rat has two pairs of limbs. Each of the front legs has four toes, each with a claw. Each of the hind legs has five toes, each with a claw. On the head are two ears with pinnae directed to the front. At the front of the head below the ears are two eyes, each with a movable upper and lower eyelid and a nictitating membrane. At the tip of the pointed snout are two extenal nostrils just above the split upper lip of the mouth. On each side of the split upper lip are long sensitive whiskers. The long tongue acts as an organ of taste and helps with eating. Both the upper and lower jaws have teeth situated in sockets. The neck is short and thick.
On the ventral surface of the female's trunk there are six pairs of teats. The genital opening (vagina), anus and and urinary opening are separate in the female and are found between the hind legs. In the male the urinary and sexual opening open jointly at the tip of the penis. The scrotum holds the testes outside the body between the hind legs. The tail is long and ends in a thin point.
During copulation the penis of the male is inserted into the vagina of the female. The sperms are discharged and and fertilisation occurs internally in the oviducts. The zygote develops into the embryo in the uterus of the female. The placenta, an intimate physiological connection between the embryo and the mother, supplies food and respiratory gases and removes wastes and carbon dioxide from the embryo. After a gestation period the young are...