LOCOMOTION. 57 



again. Most of the bivalve mollusks, such as the clams, 

 move from place to place, in a similar way. A fleshy 

 organ, called the foot, is thrust forward, and its extremity 

 fixed in the mud or to some firm object, when it contracts, 

 and thus draws along the body, and the shell enclosing it. 

 The snails and many similar animals have the under surface 

 of the body composed of an infinitude of very short muscles 

 which, by successive contractions, so minute indeed as 

 scarcely to be detected, enable them to glide along smoothly 

 and silently, without any apparent muscular effort. 



168. In the majority of animals, however, locomotion is 

 effected by means of organs specially designed for the pur- 

 pose. The most simple organs are the minute hair-like 

 cilicR, which cover the body of most of the microscopic 

 infusory animalcules, and which, by their incessant vibrations, 

 cause rapid movements. The sea-urchins and star-fishes 

 have little thread-like tubes issuing from eveiy side of the 

 body, furnished with a sucker at the end. By attaching 

 these to some fixed object, they are enabled to draw or roll 

 themselves along ; but their progress is always slow. In- 

 sects are distinguished for the great perfection of their or- 

 gans of motion. They have at least three pairs of legs, and 

 usually wings also. The Crustacea generally have at least 

 five pairs of legs, which are used for both swimming and 

 walking. The Worms are much less active ; some of them 

 have only short bristles at 



their sides for locomotion ; 

 and even those that have 

 numerous feet, like the cen- 

 tipedes, are not distin- 

 guished for agility. Some Fig. 33. 

 of the marine species use their gills for paddles. (Fig. 33). 



169. Among the Vertebrata, we find the greatest variety 

 of the organs and modes of locomotion, as well as the great- 



