274 VERTEBRATE ZOOLOGY 



the anus being dorsal to the urogenital opening. It contains also 

 the pelvic girdle, by means of which the skeleton of the hind limbs 

 is joined with the trunk. The forward progress of the animal 

 in locomotion is accomplished largely by means of the hind 

 limbs, the front limbs being of use principally in supporting the 

 anterior end of the body and in changing the direction of move- 

 ment. The connection of the pelvic girdle with the trunk is 

 consequently much firmer than that of the pectoral girdle; 

 the pelvic girdle forms a complete arch, the bones of which are 

 immovably joined with one another and also with the spinal 

 column by sutures. 



The Tail. The caudal region is the most flexible of all. It is 

 also of the least importance to the animal, and varies much in 

 size among rodents and other land mammals. In water verte- 

 brates the tail is a very important part of the body, inasmuch as 

 it is usually the principal organ of locomotion. Land vertebrates 

 have inherited it from their aquatic ancestors, but as it is no 

 longer of use for locomotion, it is put to a variety of other uses or 

 may be absent altogether. 



The Appendages. Two pairs of appendages are present, the 

 fore and hind limbs. Each is made up of three divisions, a prox- 

 imal, a middle, and a distal division. In the fore limb these divi- 

 sions are the upper arm, the forearm, and the wrist and hand 

 respectively ; the hand has five digits in the rabbit, each of which 

 is armed with a nonretractile claw. Note the walking-pads; 

 identify the three divisions of the limb ; also the shoulder joint 

 and elbow. 



In the hind limb the proximal division is the thigh, the middle 

 division is the shank, and the distal division is the ankle and foot, 

 the latter having four digits in the rabbit (digit number one, 

 which is medial in position when present, being absent) ; all the 

 digits are armed with nonretractile claws. Note the walking-pads ; 

 identify the three divisions of the limb, and also the heel, which 

 projects backward from the upper portion of the ankle. (For the 

 relative position of the fore and hind limbs with reference to the 

 trunk see page 210 for mammals, and page 65 for the lower ter- 

 restrial vertebrates.) 



