48 INTKODUCTION. 



which contains them, are nourished by imbibition, and mul- 

 tiply by spontaneous division. 



34. The radiated animals constitute a particular type, 

 the essential character of which consists in the form, which 

 is, a centre around which the other parts are arranged like the 

 spokes of a wheel. Their structure, rather simple, presents 

 several varieties from the simplest among them, the hydra or 

 polypus with arms, to the asterias or sea-star. They all in- 

 habit the water. 



35. The polypi form an extremely numerous class of ra- 

 diated animals. They are generally elongated, having one 

 single orifice or mouth furnished with radiating appendages; 

 they have an alimentary cavity, digest very quickly, and ab- 

 sorb by imbibition, produce buds which sometimes remain ad- 

 herent and form complicated phytoid animals, and at others 

 separate. The external and internal surfaces are alike; the 

 intermediate substances are homogeneous and gelatinous; no 

 peculiar organ is observable, except microscopical globules, 

 and they possess in a high degree the power of reproduc- 

 tion, for when divided, each part becomes an individual. 

 Light, noise, and other exterior causes, produce on them im- 

 pressions followed by motions. Some are fixed to the ground, 

 others are free. The simplest of all are those which are neck- 

 ed, as the hydra, &c.; they have a simple alimentary sac, 

 and multiply by external buds. Others again which are unit- 

 ed, excrete from their external surface a horny or calcareous 

 substance called polypier.* Finally, in others, which are 

 complicated animals, the common body envelops a secreted 

 substance, the consistence of which varies from that of jelly 

 to that of stone. 



36. The acephalous animals or sea-nettles (medusae,) 

 have a still more circular or radiated form ; they are compared 



* Under this appellation are grouped the calcareous substances 8tc. known 

 under the name of madrepore, coral, &c. which are the excretions of the po- 

 lypi here alluded to and serving them for habitation. These calcareous 

 excretions are comprehended by the French under the general designation of 

 polypier, for which there is no adequate term in the English language. 



TRASS. 



