LOCOMOTION. 81 



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

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

 in tha mud, or to some firm object, when it contracts, 

 and thus draws along the body and the shell enclosing 

 it. Snails, and many similar animals, have the fleshy under 

 surface of their 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 are the minute, hair-like cilia, 

 which fringe the body of most of the microscopic infu- 

 sory animalcules, and which, by their incessant vibrations, 

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

 have little thread-like tubes issuing from every 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. Insects 

 are distinguished for the number and great perfection of their 

 organs of motion. They have at least three pairs of legs, 

 and usually wings also. But those that have numerous 

 feet, like the centipedes, are not distinguished for agility. 

 The Crustacea generally have at least five pairs of legs, 

 which are used for both 



swimming and crawling. 

 The Worms are much less 

 active; some of them have 

 only short bristles at their 

 sides. Some of the marine 

 species use their fringe-like gills for paddles. (Fig. 33.) 



169. Among the Vertebrata, we find the greatest diversity 

 in the organs of locomotion and the modes of their applica- 

 tion, as well as the greatest perfection, in whatever element 



