36 THE TEREBRATULA. 



to receive the animalcules, or minute beings, upon which 

 the mollusk subsists, and which are carried to it by the 

 current. 



It has been said, that the arms of one species of 

 terebratula are united to certain complicated elevations 

 on the inner surface of one of the valves ; this is called 

 imperforate : and it may here be observed that, of the two 

 valves inclosing the animals of this genus, one valve, more 

 convex than the other, is perforated at 

 the top to give passage to the peduncle^ 

 covered with a prolongation of the 

 mantle, by which the animal is at- 

 tached to the rock. 



The brachiopods exhibit a curious 



Terebratula trimcata. 



modification of the respiratory or aerat- 

 ing apparatus peculiar to themselves, and which has 

 induced some naturalists to term them palliohranchiata ; 

 that is, mantle-gilled animals ; for, instead of pos- 

 sessing an apparatus exclusively devoted to this im- 

 portant function, as the other mollusca have, the mantle 

 itself is adapted for this office. It is traversed by large 

 vessels, and fringed all round its circumference, as seen 

 in the preceding figure of the lingula, with vibratile 

 filaments, c c, which by their action drive a current 

 of water over the mantle, on which the vessels ramify, 



