580 BRACHIOPODA. 



required, and to the fact that the vascular system is essentially an 

 organ separate from the coelom and from other organs derived from 

 the primitive enteron, it cannot be accepted (it should not have been 

 made at the present day by any well-informed Zoologist) without a 

 good deal more evidence than has yet been adduced in its favour. 



The body-cavity is well developed, and is a coelom. As already 

 stated it is divided, incompletely it is true, into two lateral halves by 

 a longitudinal mesentery, and into three successive chambers by two 

 imperfect transverse septa, the 'gastroparietal and ilioparietal bands. 

 The latter structures are, however, merely bands, there being three 

 (or two) gastroparietal and two ilioparietal. They are important, 

 because they may turn out to be remains of transverse septa 

 dividing up the coelom in the embryo (see account of development). 

 The body-cavity contains a corpusculated fluid, and possesses a 

 ciliated epithelial lining. It is prolonged into the mantle lobes as 

 i\\Q pallial sinuses, which may be much branched (Lingula). With 

 regard to other extensions of the body-cavity we know very little. 

 For instance, there are two channels running along each arm, the one 

 small and in relation with the row of tentacles, into each of which it 

 sends a prolongation, and the other larger and in relation with the lip 

 (1 epistome). It is disputed whether these canals communicate with 

 each other or with the body-cavity. It is said, however (see above), 

 that the tentacular channel is a blood vessel, and is a continuation of 

 the vessel which leaves the anterior end of the heart. 



As in coelomate animals the renal organs are nephridia. There 

 is usually one pair, but in Rhynclionella there are two. They open 

 externally on each side of the mouth, near the base of the arms, and 

 internally by funnel-shaped apertures into the body-cavity. When 

 there is one pair, the internal opening is on the ilioparietal band, into 

 the posterior chamber of the body-cavity ; when there are two pairs 

 the openings of the second pair is supported by the gastroparietal 

 bands, and is into the middle chamber of the body-cavity. The 

 external openings of the posterior pair, i.e. the pair usually present, 

 are a little on the ventral side of the mouth, while those of the 

 anterior pair are a little nearer the dorsal valve. The nephridia 

 function also as generative ducts, and were called oviducts by 

 Hancock. 



Generative organs. In all probability most Brachiopoda are 

 dioecious, but possibly a few of them may be hermaphrodite. The 

 generative organs are developments of the coelomic epithelium. 

 They are thick yellow bands or ridges which project into the body- 



