110 OF LOCOMOTION. 



brought forwards to repeat the same process. Most of the bi- 

 valved mollusca, 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 en- 

 closing it. Snails, and many similar animals (fig. 35), have the 

 fleshy under-surface of their body (a, 6) composed of an infinitude 

 of very short muscles, which, by successive contractions so mi- 

 nute, indeed, as scarcely to be detected enable them to glide 

 smoothly and silently along, withoutany apparentmuscular effort. 



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

 effected by means of organs specially designed for the purpose. 

 The most simple are the minute hair-like cilia, fringing the 

 body of most of the microscopic infusory animalcules (fig. 

 171), and which, by their incessant vibrations, cause rapid move- 

 ments. The sea-urchins (fig. 174) and star-fishes (fig. 36) have 

 little thread-like tubes issuing from every side of the body, fur- 

 nished 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 (fig. 34), and usually two pairs 

 of wings (fig, 369), 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 gills for paddles. 



234. Among the vertebrata, we find the greatest diversity 

 in the organs of locomotion, and the modes of their application, 

 as well as the greatest perfection, in whatever element they may 

 be employed. The sailing of the eagle, the bounding of the 

 antelope, the swimming of the shark, are not equalled by any 

 movements of insects. Tli:3 superiority is due to the internal 

 skeleton, which, while it endows tlt j.\.iniiil with great force, gives 

 to the motions, at the same time, a nice degree of precision. 



[ 235. Before entering upon the study of the various mo- 

 tions of the vertebrate animals, and the means by which these 

 are performed, it is important to put the student in posses- 

 sion of a standard by which he will be enabled to compare the 

 form of the osseous elements and the modifications they undergo 

 in fishes, reptiles, birds, and mammals. With this view we 



