326 



POPULAR CONCHOLOGY. 



soft tube, which perhaps is to be regarded as a prolonga- 

 tion of the mantle, and contains within it fibres either 

 muscular or tendinous. 



The nervous system is not yet sufficiently known, but 

 two or three ganglia have been discovered surrounding 

 the oesophagus. Moreover no organs of sense are known 

 to belong to them, besides the cirrhi which act as feelers, 

 on the edges of the mantle, and upon the arms. The 

 digestive apparatus of the Brachiopoda does not differ 

 essentially from that of other Molluscs. The intestinal 

 canal begins with a simple opening concealed between the 

 bases of the two tentacula, whence in the Terebratula 

 proceeds a tolerably long, curved (or contorted) oesophagus, 

 which leads into a roomy stomach, whilst in the rest of 

 the Brachiopoda the intestinal canal is continued without 

 any distension, making one or more convolutions. Sali- 

 vary glands do not exist, but the liver is readily distin- 

 guished in the form of little fascicles of glands ; it pours 

 its secretion immediately into the stomach. 



The system of blood vessels is very curious : the veins 

 that return from the mantle gills do not unite into a 

 single heart, but pour their contents into two separate 

 hearts, lying to the right and left of the intestinal sac; 

 by the contraction of these hearts, the blood is discharged 

 freely without the intervention of vessels, into the intes- 

 tinal cavity. The inner lining of the mantle serves the 

 office of gills ; it is furnished with a very complete system 

 of blood vessels. In the genus Lingula the gill vessels 

 are contained in a kind of tumid prominences, which give 

 to the inner surfaces of the mantle a very peculiar ap- 

 pearance. 



The structure of the shell has many peculiarities. In 

 the Discina it is almost entirely horny. In the Lingula 

 there is a marked epidermis, which is entirely absent in 



