70 ORGANIC FUNCTIONING 



the clutching movements of the toes of the birds. There is exact 

 co-ordination of the right and left limbs. 



Quadrupedal locomotion is exhibited by most mammals ; by 

 most reptiles and many amphibia. There are four limbs, in 

 two pairs. Fore and hind limbs act similarly (except as indi- 

 cated below) and there is co-ordination between anterior and 

 posterior pairs. Individually the limbs act much as in bipedal 

 locomotion. 



Multipedal locomotion is exhibited by many insects, Crustacea, 

 spiders, mites, worms, etc. In such cases the animal body is 

 clearly segmental and each segment typically carries a pair of 

 appendages. There may be anything from three to several dozen 

 pairs of appendages. In the arthropods the latter are jointed and 

 in these animals the motions of the " walking legs " are essentially 

 similar to those of the quadruped mammals — that is from a 

 mechanical point of view. Complete co-ordination is implied 

 when there are many pairs of appendages. 



Flight. In the cases of the birds, the mammals and the extinct 

 flying reptiles the organs of flight are morphologically similar to 

 the limbs of the quadruped. In the birds the fore limb is 

 feathered ; in the mammals that fly (the bats) a light membrane 

 is stretched between the fore and hind limbs and the body, and 

 in the flying reptiles the membrane was stretched between the 

 digits. In the insects the one or two pairs of wings are special 

 appendages, morphologically dift'erent from the wings of birds 

 and mammals. The wings are air-planes used in gliding (in the 

 cases of the large birds) and, of course, as propellers also. They 

 beat on the air with rapid oscillatory motions in the cases of the 

 small birds and insects. In flying animals there are marked 

 modifications of the other parts of the body. Thus the hollow 

 and light bones of the bird, the shape of the body and the high 

 efficiency of the respiratory organs. 



Swimming. Most kinds of animals swim in water, either in- 

 stinctively or by trial. In mammals, reptiles and amphibia the 

 swimming organs are the limbs, used so as to propel the body in 

 water. The birds rather paddle with the hind limbs, floating 

 high out of the water and also diving. The fishes use the paired 

 fins (which are morphologically similar to the limbs of mammals), 

 but these organs are relatively inefficient compared with the tail : 

 the median fins certainly, and the paired fins to some extent, act 



