366 



BRACHIOPODA. 



numerous ducts derived from the hepatic follicles open by large 

 orifices. The structure of the liver in these animals is displayed 

 by Professor Owen in the memoir from which the annexed figures 

 are taken, and the simplicity of its organization affords an interest- 

 ing lesson to the physiologist. The hepatic organ (Jig- 172, a, c) 

 consists essentially of numerous secerning caeca (Jig. 172, B), as yet 

 easily separable from each other ; over which the visceral blood-ves- 

 sels ramify, and bring to the secreting sacculi the circulating fluid 

 from which the bile is elaborated. 



(400.) The greatest peculiarity observable in the structure 

 of the Brachiopoda is seen in the arrangement of the respiratory 

 system ; for these animals, instead of possessing proper branchial 

 organs as is the case with all other Mollusca, have the mantle 

 itself converted into a respiratory surface, and traversed by the 

 ramifications of large blood-vessels, which form an elaborate arbor- 

 escence spreading through its texture, so that it is obviously well 

 adapted to perform the office assigned to it ; more especially as its 

 circumference is thickly studded with vibratile cilia, disposed in such 

 a manner that by their ceaseless movements they impel continued 

 supplies of aerated water Fig. 173. 



over the whole of this vas- 

 cular membrane. The 

 lobe of the mantle which 

 lines the perforate valve 

 of Terebratula Chilensis 

 (fig> 173, c) contains four 

 large longitudinal venous 

 trunks (m, m), and two 

 others of similar dimen- 

 sions are seen in the op- 

 posite lobe a. These veins 

 take their origin by innumerable radicles from a circular canal of 

 great delicacy which encompasses the entire circumference of the 

 mantle (d) ; and it is in this canal that Mr. Owen supposes the 

 branchial arteries that may be seen to accompany the veins above 

 described terminate. The four veins which are placed in the per- 

 forated lobe of the mantle form two trunks near the visceral mass ; 

 and these, joining those of the opposite lobe, terminate in two dis- 

 tinct contractile cavities, or hearts, seen near the exterior margin of 

 the liver. The arms of the Brachiopoda, notwithstanding their 

 gill-like structure, seem to have nothing to do with the renovation 



