BRACHIOPODA. 



365 



transverse connecting bar, where they again advance forwards, and 

 terminate by making a half-spiral twist in front of the mouth. 



One use assignable to the spiral arms of the BRACHIOPODA 

 is no doubt connected with the opening of the shell, which, in 

 species provided with muscular and retractile organs of this descrip- 

 tion, is mainly effected by their forcible protrusion. In Terebra- 

 tula Chilensis, however, and other species in which the arms are 

 not extensile, Mr. Owen conceives that the elaborate internal 

 framework above described answers a similar purpose ; observing, 

 that the muscular stem, by means of its attachment to the calcareous 

 loop, has the power of acting upon that part to the extent its elasti- 

 city admits of, which is sufficient to produce such a degree of convex- 

 ity in the reflected portion of the loop as to cause it to press upon 

 the perforated valve and separate it slightly from the opposite one.* 



(399.) The most obvious function, nevertheless, attributable to 

 the tentacular organs of the animals composing this class is connected 

 with the procurement of food ; for, being utterly deprived of pre- 

 hensile instruments, without some adequate contrivance these help- 

 less creatures, imprisoned in their testaceous covering, and fixed 

 immovably in one locality, would be utterly unable to obtain the 

 nourishment necessary for their support. The provision for this 

 purpose is found in the arms, whether they be extensible or attach- 

 ed to calcareous loops ; for these 

 organs, being covered by cilia, pro- 

 duce powerful currents in the sur- 

 rounding medium, which, being di- 

 rected towards the mouth as to a 

 focus, hurry into the oral aperture 

 whatever nutritive particles may 

 chance to be in the vicinity. The 

 mouth itself is a simple orifice with 

 prominent fleshy lips (Jig. 170, I), 

 but unprovided with any dental ap- 

 paratus. The alimentary canal in 

 Lingula is a long and convoluted 

 tube, but without a perceptible sto- 

 machal dilatation ; in Terebratula, A 

 however, there is a large oval stomach (fig. 172, A, d), into which 



* Innumerable shells of extinct species of Brachiopoda occur in a fossil state ; and in 

 many of them (Spirifera, &c.) an internal framework, analogous in some respects to 

 that described in Terebratula Chilensis, is discernible. 



Fi 



