RESPIRATORY AND CIRCULATORY SYSTEMS. 



513 



(1347.) The Infusoria appear to be the chief food of all the Brachio- 

 pods, perhaps the only food of the articulated species. In the latter, 

 siliceous cases of the Diatomacece are almost always found, and sometimes 

 in abundance. Lingula, however, appears to be a more general feeder, 

 its intestine frequently containing a great variety of matter. 



(1348.) 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 arborescence spreading through its 

 texture, so that it is ob- 

 viously well adapted to Fig. 259. 

 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 cease- 

 less movements they impel 

 continued supplies of 

 aerated water over the 

 whole of this vascular 

 membrane. The lobe of 

 the mantle which lines 

 the perforate valve of 

 Terebratula Chilensis (fig. 

 259, c) contains four large 

 longitudinal venous trunks or sinuses (m m), and two others of similar 

 dimensions are seen in the opposite lobe (). These sinuses take their 

 origin by innumerable radicles from a circular canal of great delicacy 

 which encompasses the entire circumference of the mantle (d). 



(1349.) A heart is present in all the Brachiopoda; and when in an 

 expanded state, it is of considerable size. In the articulated species, it 

 is appended to the middle line of the stomach, and projects freely into 

 the perivisceral cavity, reaching down almost to the anterior margin of 

 the oviducts. Its walls, when expanded, though rather thin, are firm, 

 and do not collapse. They are composed of two layers, the inner of 

 which is distinctly muscular, the fibres running in various directions, 

 but principally radiating from centres ; the outer layer is transparent 

 and homogeneous. The interior is devoid of columnae earner, and per- 

 fectly smooth. When this organ is in a contracted state, its size is very 

 much reduced, the surface is slightly wrinkled, and the walls much 

 thickened. 



(1350.) This unilocular heart in Waldheimia australis receives a 



2 L 



Vascular system of Terebratula Chilensis : a, c, the 

 mantle ; d, circular canal encompassing the margin of 

 the mantle ; f, g, h, muscles of attachment ; mm mm, 

 large venous trunks in the mantle. 



