APPARATUS OF MOTION. 75 



stantial framework for the body, which has been variously 

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

 carapace, skeleton, Sfc. The study of these parts is one of the 

 most important branches of comparative anatomy. Their 

 characters are the most constant and enduring of all others. 

 Indeed, these solid parts are nearly all that remains of 

 the numerous extinct races of animals of past geological 

 eras ; and from these alone are we 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 assuming the form of a simple inter- 

 nal skeleton, or forming extensively branched stems, as 

 in the sea-fans ; or giving rise to solid masses, furnished 

 with numerous cavities opening at the surface, from which 

 the movable parts of the animals are protruded, with the 

 power, however, of retracting themselves at pleasure, as in 

 the corals. In the Echinoderms, the test is intimately con- 

 nected with the structure of the soft 



parts. It is composed of numer- 

 ous little plates, sometimes con- 

 solidated and immovable, as in 

 the sea-urchins, (Fig. 26,) and 

 sometimes so combined, as to 

 allow of various motions, as in 

 the star-fishes, (Fig. 17,) which use their projecting rays, 

 both for crawling 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 afford complete protection to the animal 

 within their cavities. In a few, the shell is too small for this 

 purpose ; and in some it exists only at a very early period, 



