LESSON 78.] NERVOUS SYSTEM IN THE BRACHIOPODA. 257 



their summit. A large and very capacious vein (c, c) runs through 

 each side of the mantle, which divides into a great number of smaller 

 branches, distributed to the outer edge of the mantle. In the centre 

 of each of these veins, an artery (e, e) is distributed, and ramifies 

 everywhere upon their surface. In addition to the vascular system, 

 nervous filaments (f ) are extensively distributed ; they are given to 

 the muscles, and pass directly through the mantle to its edge. The 



FIG. 376. 



Dorsal mantle of Terebratula Austral!?. 



sexes are distinct in Terebratula ; thus the Ovarium, filled with eggs, 

 is seen at g, and the external thickened margin of the mantle at h. 



1167. Having to breathe under such an enormous weight and 

 pressure of water, something more than the ordinary form of respi- 

 ratory apparatus becomes essentially necessary. 



1168. There are no special organs of respiration in these ani- 

 mals ; that function appears to be performed by the singularly vascu- 

 lar mantle, but accessory thereto is, doubtless, the very powerful 

 arms, with the no less powerful vibratile cilia. By their action, the 

 surfaces of the mantle are kept constantly laved with water for res- 

 piration, and as the fluid always makes a circuit in one uniform di- 

 rection in this, as in other bivalves, the food which is contained in it 

 is conveyed to and appropriated by the mouth, which is abundantly 

 supplied with its own vibratile organs for that purpose (Figs. 256 and 

 257, a, a). 



17 



