LECTURE II. 67 



structure, and mode of connection, should 

 divide and determine the motions to parti- 

 cular parts. These structures are differently 

 placed in different animals. In the earth- 

 worm, caterpillar, and nereis, the firm 

 structure is the skin, which is divided into 

 rings, all of which have motion on each 

 other, and into these are muscles inserted, 

 so that a variety of particular motions is 

 produced. In insects, spiders, anr /lobsters, 

 whose firm parts are more complicated and 

 answer a greater variety of purposes, we 

 find the fixed parts still more dense. In 

 the two former they are of a horny, and in 

 the latter of a calcareous nature. These 

 parts also make a covering for the animal, 

 but there are processes of them going deep 

 for the attachment of muscles. In tor- 

 toises, lizards, snakes, &c. though they have 

 firm external coverings, yet there is an in- 

 ternal apparatus for motion, which is prin- 

 cipally bone. In the more perfect animals, 

 external firm parts are rarely found. They 

 have an internal osseous skeleton, so that 

 the muscles and their attachments are re- 

 versed ; and thus we find them arranged 



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