APPARATUS OF MOTION. 51 



increase the force and precision of the movements. The 

 solid parts are usually so arranged as to form for the body 

 a substantial frame work, which has been variously desig- 

 nated in the several classes of animals, as the test, shell, 

 carapace, skeleton, fyc. The study of these solid parts con- 

 stitutes the most important branch of comparative anatomy. 

 Their characters are the most constant and enduring of all 

 others. Indeed, these solid parts are all that remains to us of 

 the numerous extinct races of animals of past geological 

 eras ; and from these alone, we are to determine the struc- 

 ture and character of the ancient fauna. 



154. Most of the Radiata have a calcareous test or crusty 

 shell. In the Polypi, this structure, when it exists, is usually 

 very solid, sometimes in the form of a simple internal 

 stem, or extensively branched, as in the sea-fans ; and 

 sometimes in solid masses, furnished at the sides with nu- 

 merous cavities, in which the animals are lodged, with the 

 power, however, of protruding and retracting themselves at 

 pleasure, by means of their muscles, as in the corals. In 

 the Echinoderms, the test is brittle, and intimately united 

 with the soft parts. It is composed 



of numerous little plates, some- 

 times consolidated and immovable, 

 as in the sea-urchins, (Fig. 26), 

 and sometimes so combined, as to 

 allow of various motions, as in the Fig. 26. 



star- fishes, (Fig. 17), which use their arms both for crawl- 

 ing and swimming. 



155. In the Mollusks, the solid parts are secreted by the 

 skin, most frequently in the form of a calcareous shell of 

 one, two, or many pieces, serving for the protection of the 

 soft parts which they cover. These shells are generally so 

 constructed as to allow the animal to retire and conceal 

 itself completely within their cavities. In a few, the shell 



