598 COMPARATIVE STUDY OF MOTION. 



the powerful lateral muscles. Usually the caudal fin is bent in two opposite 

 directions above and below; in slight movements it is bent only in one direction. 

 By sudden extension of the tail, the fish exerts a pressure against the water, 

 and thrusts itself forward. Many fish, such as the salmon, can thus hurl them- 

 selves up out of the water. The dorsal and anal fins maintain the vertical posi- 

 tion. The pectoral and abdominal fins, corresponding to the extremities, effect 

 the smaller movements, especially upward and downward; during sleep the ab- 

 dominal fins are spread out. Most fish possess a swimming-bladder. This is 

 wanting, however, in many cartilaginei (cyclostomi) , or is rudimentary, as in 

 the shark. It either opens into the alimentary tract through the air-passage, or 

 the latter is only a temporary structure that is later obliterated. The swimming- 

 bladder is, in part, to be regarded as a respiratory organ with afferent and efferent 

 vessels, while in part it serves for hydrostatic purposes. In the dipnoi the bladder 

 is transformed into a lung. The body of swimming birds has a much lighter 

 specific gravity than has water, while their feathers are lubricated by the coccygeal 

 glands. They propel themselves forward with their webbed feet. 



Flying, in mammals, is confined to the bat and its allied species. The bones 

 of the upper extremities, including the phalanges, are greatly lengthened. Be- 

 tween the latter, as well as the hind limbs (except the feet) , is stretched a thin 

 membrane, which also partially includes the tail. The flying movement of this 

 membrane is effected by the powerful pectoral muscles, which arise in part from 

 a ridge-like elevation of the sternum and the strong clavicles. The so-called 

 flying lemurs, squirrels, and opossums have merely a duplication of the skin, 

 stretched laterally between the larger bones of the extremities, and serving 

 as a parachute in jumping. 



Man is unable to imitate flying movements successfully, for even though he 

 were able to construct artificial wings, he would still lack the strength of the 

 pectoral muscles that is necessary to effect elevation of the body. 



In birds the body specifically is exceedingly light. Large air-sacs extend from 

 the lungs into the thoracic and abdominal cavities ; even the bones are connected 

 with the lungs by special canals, so that all the spaces in the bones of the cranium, 

 spinal column, bill, and extremities are filled with air instead of marrow. The 

 upper extremities, transformed into wings, are supported by the powerful coracoid 

 bone and the clavicles (furcula), the latter being fused in the middle. The wings 

 are operated by the powerful pectoral muscles, which arise from the large crest 

 of the sternum. 



In flying upward the wings are half closed, and are moved with the anterior 

 border directed obliquely forward and upward. The plane of the wings, without 

 offering resistance to the air, follows in the same direction as the edge of the 

 wings. Then the latter are spread out in a large arc downward and backward, 

 with their surfaces pressed downward. While the under surfaces of the wings 

 press against the air from above and forward, downward and backward, the bird 

 moves forward and upward. Birds can rise only against the wind, partly because 

 the wind striking horizontally against their backs would press them down, and 

 partly because it would disarrange their feathers. By means of a revolving photo- 

 graphic camera, arranged in an apparatus resembling a musket, Marey obtained 

 complete series of pictures of flying birds at which he directed the apparatus. 



Among invertebrates, all insects possess six legs. In addition some of them 

 (butterflies, bees) have two pairs of wings on the second and third thoracic 

 segments. In beetles and earwigs the first pair is merely a covering; in the 

 strepsiptera it is entirely rudimentary. Conversely, in the flies the second pair 

 of wings is reduced to small swinging bulbs. Lice, fleas, and bedbugs have no 

 wings at all. All spiders have eight legs, the moths having six in their youth. 

 In the centipedes the first three body-rings carry each one pair of legs, while 

 all the rest have either one or two pairs. The crustaceans also possess numerous 

 feet, as a rule, some of them undergoing peculiar transformations, for example 

 in the river-crawfish into mandibles, claws, ambulatory feet, abdominal swimming 

 feet and fin-foot. In the arthropods all of the muscles are inserted on the inner 

 surface of the chitinous covering. The muscles themselves are highly developed 

 and capable of a great amount of energy and rapidity of movement. 



Molluscs lack internal supporting organs, while external ones (shells) of simpler 

 construction are present. The muscles, which are partly striated , form a musculo- 

 cutaneous tube about the body that causes the changes in the form of the body. 

 In mussels the strong single or double sphincter-muscle of the shells is noteworthy. 

 In the pecten (scallops) this muscle effects a springing movement in the water 



