ORGANS OF LOCOMOTION , FOE WATER LIFE 305 



Arrangement of muscle fibers. — The muscles of all ani- 

 mals lower than the vertebrates are parallel fibers, and are 

 never united into bundles like those of man. In no case 

 are they found bound together by a connective tissue, or 

 united to hard parts by tendons. 



All the muscles of the starfish, worm, and oyster 

 family are of the unstriated variety. All those of the 

 lobster, spider, and insect tribes (Arthropoda) are stri- 

 ated. 



Organs of locomotion. — In many of the worms and 

 lower forms, and even in some of the higher forms, such as 

 the snake, motion is secured by undulations of the body 

 as a whole. Most animals, however, have special organs of 

 locomotion. The number of these organs varies, but, in 

 general, the higher we go in the animal scale the greater the 

 perfection and the fewer the number. Thus, while a star- 

 fish has hundreds of tube feet, and a centiped many legs, 

 the two arms and legs of man are much more effective 

 as means of progress. While it is impossible to consider 

 in detail all the organs of locomotion in the animal world, 

 we may consider for a moment the characteristics common 

 to those adapted for special surroundings. All the organs 

 of locomotion about to be described depend upon the action 

 of variously arranged muscle layers, that is their move- 

 ments are muscular movements. 



Organs of locomotion for water life. — Since the water 

 supports nearly all the weight of the body, we should 

 expect to find the organs of locomotion of aquatic animals 

 ill adapted for supporting the body. For example, the 

 cilia of the protozoan, or the fins of the fish, are not in- 

 tended to support the body, but to push it through the 

 water; while even the webbed feet of the duck are placed 



BDDT. PHTS.— 20 



