474 RECENT BRACHIOPODA chap. 



tube, opens into the tentacular canal, and consequently supplies 

 the tentacles with blood. These two canals which diverge 

 from the median artery are connected ventrally by a vessel which 

 runs below the oesophagus; the latter is therefore surrounded 

 by a vascular ring. Blochmann also describes two pairs of 

 vessels that were seen and figured by Hancock. A pair of 

 these pass over the gastro-parietal mesenteries and into the 

 dorsal mantle sinus, the second pair pass over the ileo-parietal 

 mesenteries and into the ventral mantle sinus ; each of these four 

 arteries runs to one of the four generative glands, which, as is so 

 usually the case in the animal kingdom, have thus a specially 

 rich blood supply. If this description should prov^e to be correct, 

 the vascular system of Brachiopods shows a striking resemblance 

 to that of the closed vascular system of the unarmed Gephyrea, 

 except that the former group has specialised genital vessels. 

 The blood is colourless. 



Joubin's description of the vascular system of W. venosa differs 

 in some respects from that of Blochmann. He regards the heart 

 as collecting the lymph which it receives from numerous lacunar 

 spaces in the walls of the alimentary canal, and distributing it 

 through various vessels, which in the main correspond with those 

 of Blochmann, and which run both to the "arms" and to the 

 generative glands. The latter vessels, however, open freely 

 into the body cavity, and the fluid which is forced out from 

 their openings freely bathes the organs found in the body 

 cavity. Whichever of these accounts should prove to be more 

 closely in accordance with the facts, there is little doubt that in 

 addition to the true blood there is a corpusculated fluid in the 

 body cavity which is to some extent kept in motion by the 

 ciliated cells that line its walls. 



The Excretory Organs 



The excretory organs (kidneys) which were at one time 

 regarded by Cuvier and Owen as hearts, are typical nephridia — 

 that is to say, they are tubes with glandular excretory walls 

 which open at one end by a wide but flattened funnel-shaped 

 opening into the body cavity, and at the other end by a circular 

 pore to the exterior (Fig. 314). In Rhynchonella^ where 

 there are two pairs of these tubes, — the only evidence that the 

 group presents of any metameric repetition of parts, — the inner 



