100 



THE SKELETON OF POLYPS. 



218. In the great majority of animals, motion is aided 

 by the presence of solid parts, of a bony or horny structure, 

 which either serve as firm attachments to the muscles, or, 

 being arranged to act as levers, they increase the force and 

 precision of the movements. The solid parts are usually so 

 constructed as to form for the body a substantial frame-work, 

 which has been variously designated in the several classes of 

 animals, the test, shell, carapace, and skeleton. The study 

 of these parts is one of the most important branches of com- 

 parative anatomy, as their characters are the most constant and 

 enduring of all others. Indeed, these solid parts are nearly all 

 that remain to us of the numerous extinct races of animals of 



Fig. 69. 



Figs.69 and 70.-- 



past geological eras ; 

 and from these a- 

 lone, we are en- 

 abled to determine 

 the structure and 

 character of the an- 

 cient fauna. 



219. Most of 

 the radiata have a 

 calcareous test or 

 shell. In the po- 

 lyps, this structure, 

 when it exists, is 

 usually very solid, 

 sometimes assum- 

 ing the form of a simple inter- 

 nal skeleton, or forming exten- 

 sively branched stems, as in the 

 sea-fans; and sometimes solid 

 masses, furnished at the sides 

 with numerous cavities, in which 

 the animals are lodged, with the 

 power, however, of protruding 

 and retracting themselves at 

 pleasure, by means of their mus- 

 cles, as in the corals. 



[Litharcea Welsteri (fig. 69) 



is a fossil coral, from the ter- 



. tiary sands of Bracklesham Bay, 



