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DIVISION OF LABOR IN VERTEBRATE GROUP 109 



the mouth, is surrounded by a circle of tentacles, delicate organs 

 which aid in getting food. There are only two layers in the body : 

 the outside layer of cells is primarily protective ; the inner layer 

 is chiefly digestive in function, though movement, locomotion, 

 sensation, and reproduction are taken care of by various types 

 of cells. Here, in other words, physiological division of labor is 

 simple and the body structures also are simple. In so-called 

 " higher " animals, as in a fish, frog, or bird, the organs are more 

 complicated as division of labor becomes more specialized. In 

 the human arm, an organ fitted for certain movements, think of 

 the number of tissues and the complicated actions which are 

 possible. The most extreme division of labor is seen in the or- 

 ganism which has the most complex actions to perform, with each 

 organ fitted for its own particular work, which it does quickly 

 and well. 



Division of Labor in the Vertebrate Group ; the Frog. — Al- 

 though we are interested chiefly in a study of man, because we wish 

 to prepare ourselves for efficient citizenship, it is easier to study in 

 the laboratory some small animal that resembles man in general 

 structure. Such an animal is the frog, for it is a typical representa- 

 tive of the vertebrate or backboned animals, and it is easy to obtain 

 and to handle. 



The frog's internal organs are somewhat like those of man. 

 Thus the frog has lungs, a heart, a stomach, intestine, various 

 glands, etc. 



You will notice that the appendages of a frog have the same 

 general position on the body and the same number of parts as do 

 your own (upper arm, forearm, and hand, thigh, shank, and foot, 

 the latter much longer relatively than your own) . Note that while 

 the frog's hand has four fingers, the foot has five toes, the latter 

 connected by a web. In swimming most of the frog's energy is 

 used in the powerful backward push of the hind legs, which in a 

 resting position are held doubled up close to the body. On land, 

 locomotion may be by hopping or crawling. 



Adaptations for life in the water are numerous. The ovoid 

 body, the head merging into the trunk, the slimy covering (pro- 

 vided by mucus cells in the skin), and the powerful legs with webbed 

 feet, are all evidences of the life which the frog leads. 



