108 ESSENTIALS OF ZOOLOGY 



of the abundance of available food and shelter. Natural enemies of 

 the rat are dogs, cats, coyotes, foxes, minks, weasels, skunks, owls, 

 hawks, king snakes, black snakes, and bull snakes. 



External Structure of the Rat 



The rat is a quadruped and is quite well adapted for climbing as 

 well as for walking. The body, with the exception of the tail, is 

 covered with thick fine fur mixed with stiff hairs. The heavy hairs 

 or '^feelers" about the face are called vibrissae. Those around the 

 mouth are known as buccal vibrissae and are divided into two 

 groups: the mystacial in several rows above the upper lip in front 

 of the eye, and the submental group on the chin. Above the eye is 

 the superciliary group and the single bristle on each side of the face 

 below the outer corner of the eye is a genal vibrissa. The interramal 

 group is a tuft in the ventral midline between the angles of the 

 jaws. All of these bristles are very sensitive to touch and are of 

 great use to an animal whose habits are primarily nocturnal. 



The body of the rat may be divided into four regions : head, neck, 

 trunk, and tail. In addition, there are the two pairs of appendages. 



The head is rather pointed at the anterior end, and located termi- 

 nally are the two nostrils, each shaped like an inverted comma and 

 capable of being closed in case the animal goes under water. 



The naked skin around the nostrils is very sensitive to touch. The 

 mouth is located in a subterminal position. There is a cleft in the 

 upper lip which exposes the incisor teeth, even when the mouth is 

 closed. The two sides of the cleft extend inward toward the mid- 

 line behind the upper incisors almost to meet each other and prac- 

 tically exclude these teeth from the buccal cavity proper. In the hare 

 this cleft in the upper lip is very prominent, and it is from this that 

 the abnormally cleft lip in man has come to be known as ^'harelip." 

 The eyes of the rat are small with relatively large pupils. They are 

 well set in the lateral orbits and protected by upper and lower eye- 

 lids. There is a nictitating membrane, supported by a semilunar 

 cartilage, in the medial corner of the eye. The eyelashes are fine and 

 short. The eyeballs are kept moist by secretions from Plarderian 

 and lacrimal glands, spread over their surfaces by winking. The 

 ears are located well posterior on the head. The pi^ma which is the 

 conspicuous part of the external ear is large, erect, and supported 



