PALLIOBRANCHIATA ; GENERAL CHARACTERS. 



421 



numerous vibratory filaments, for the more certain capture of the 

 prey, and probably also for assisting in the maintenance of the 

 respiratory current an extraordinary provision, which is ren- 

 dered necessary by the great depth at which these animals live, 

 and the consequent enormous pressure of the water around. All 

 the existing genera of this Class are attached, in some way or 

 other, to solid bodies. In Terebratula and Lingula, this attach- 

 ment is effected by means of a fleshy tubular footstalk ; and this 

 footstalk or peduncle passes out, in the Terebratula and its allies, 

 through an aperture or notch in the beak of the lower valve of 

 the shell (Fig. 700). In Crania and its allies, on the contrary, 



FIG. 699. RHYNCHONELLA 

 P8ITTACEA. 



FIG. 700. LOWER VALVE OF 

 TEREBRATULA. 



the peduncle is wanting : and the lower valve of the shell itself 

 becomes the medium by which the attachment of the animal to 

 the rock is accomplished. There is thus the same kind of dif- 

 ference between these genera, as between the pedunculated and 

 sessile Cirrhopods, a group, of which we are also strongly 

 reminded by the structure of the arms of the Brachiopoda. 



1049. The valves of the shell in the Palliobranchiata are 

 usually unequal, the lower (or ventral) valve being almost always 

 the largest, and projecting in the form of a beak beyond the 

 upper (or dorsal) valve. In this beak is situated the aperture 

 through which the footstalk passes. The hinge is generally 

 formed by a pair of teeth, springing from the ventral valve and 

 fitting into corresponding cavities in the dorsal valve ; there is 

 no ligament, and the valves are opened and closed by a very 

 complicated arrangement of muscles. The interior of the dorsal 

 valve very frequently bears a curious calcareous framework, 



