514 BRACHIOPODA. 



large blood- channel or vessel in front, which, running forward along 

 the dorsal ridge of the stomach, within the membrane denominated 

 mesentery, communicates on each side by several minute openings with 

 the gastric lacunce, which will be more particularly noticed hereafter. 

 The anterior extremity of this channel passes down the dorsal surface 

 of the oesophagus, and, dividing into two lateral trunks, opens at each 

 side into a system of large lacunes placed around the alimentary tube. 

 This channel is the afferent cardiac channel or branchio-systemic vein. 



(1351.) A little behind the point where the heart receives this 

 channel, two aortic vessels pass off laterally. The two orifices communi- 

 cating with these vessels are guarded by sphincter valves, resulting 

 apparently from the protrusion inwards of the lining membrane. From 

 these vessels numerous branches are given off to supply the different 

 viscera. The peripheral portion of the circulation is composed of an 

 extensive system of lacunes or blood- channels, originally described by 

 Professor Huxley, which, when the specimen is in good condition, are 

 easily traceable, the channels or spaces being then, for the most part, 

 filled with blood-corpuscles, which give to the lacunes an opake yellowish 

 hue, rendering them as distinct and sharp as though they had been in- 

 jected ; and as the circular points of union between the layers of mem- 

 brane forming their walls are transparent, and consequently liable to 

 appear of a darkish tint, they show like spots on a light ground, not 

 altogether unlike the markings upon a leopard's skin. In other lights, 

 the whole has a beautiful lace-like delicacy. These lacunes occasion- 

 ally assume a dendritic or branched character, particularly as they ap- 

 proach the margin of the mantle, where they become minute, and run 

 almost parallel to each other, forming minute twigs, which pass on to 

 the external circumference of the lobe. 



(1352.) The inner lacunes, or those of the inner wall or floor of the 

 great pallial sinuses, have a very different character ; they resemble, 

 however, the lacunes in the anterior wall of the body, with which they 

 are in communication. They are in the form of numerous narrow 

 channels, which, anastomosing at various points, compose a network of 

 very long transverse meshes ; thus most of the channels cross the direc- 

 tion of the sinuses, and run parallel to each other. Hence it would seem 

 that the walls of the body and the laminas of the pallial lobes present 

 one great system of blood- channels or lacunes, the various parts of 

 which freely communicate with each other, and surround all the viscera 

 of the body. 



(1353.) The blood-system of the brachial apparatus next claims atten- 

 tion. This is beautifully developed, and presents considerable variety in 

 the plexuses of which it is composed. The walls of the great canal, the 

 ridge supporting the cirri, the membranes that unite the upper and lower 

 membranes of the loop, and that which connects the spirals, all have 

 their systems of lacunes, which intercommunicate and form the brachial 



