THE ORBICULA — THE TUNICATA. 37 



and in which, the circulating fluid becomes subjected 

 to the oxygen. As to the general circulation of the 

 blood, and the arrangement of the nervous system, 

 little is correctly known. 



In the genus orhicida, of which one species is found 

 in our seas, the animal is inclosed in two shells, one 

 of a rounded conical figure, resembling that of a lim- 

 pet ; the other, covering the mouth of the limpet-like 

 shell, is flat, and attached to the rocks. The arms 

 of the mollusks of this genus are fringed, and curled 

 up spirally when at rest. 



It has sometimes been supposed, that animals of an 

 apparently infei'ior order, have no particular claim on 

 attention. But this is an error ; not one is there un- 

 worthy of the great Creator's power to produce and 

 to sustain it ; and, consequently, every one, however 

 humble it appears, will abundantly repay the notice 

 it receives. It is hoped that one result of the perusal 

 of the present volume will be, the direction of the 

 mind to this fact, of which it will aff'ord many illus- 

 trations. 



Among the mollusca, few animals are more singular 

 or interesting than the timicata, or, as Cuvier terms 

 them, " acephales sans coquilles." Their internal 

 structure is elaborate, and the external forms which 



