BRACHIOPODA. 497 



border of the mantle. The larger pallial vessels, and especially 

 those that terminate by closed ends in Lingula, may be regarded as 

 generative rather than branchial vessels, i, e^ as being chiefly con- 

 cerned in nourishing and affording pabulum to the generative 

 cells. In Lingula and Orbicula, the pallial trunk is continued 

 from the ventricle. The visceral chamber, in aU Brachiopods, is 

 occupied by larger and less regular venous sinuses, which seem to 

 form a complex peritoneal cavity. The sinus which surrounds the 

 intestinal canal, is figured in CCCIII. pi. \.,Jigs. 5 & 6, 8 {Lingula), pi. 

 ^> Jtff^' I5 S, (Terebratula) : the communication of this and other 

 visceral sinuses with the plicated auricle in Terebrattda is accurately 

 represented in pi. ^,Jig. 1, of the same monograph. 



The course of the circulation can only be determined by observa- 

 tion of the living Brachiopod : it may be subject to oscillations, as in 

 the Tunicata. Whatever its course, there is no actual extravasation : 

 I find, after the most patient scrutiny, no evidence of an escape of 

 the blood into mere lacunae or interspaces excavated in the tissues of 

 other and surrounding systems ; but the result of such scrutiny has 

 been, invariably, the detection of the continuation and expansiou of 

 the proper tunic of the veins into such seeming lacunae or interspaces. 

 And although, in the wide clefts between the viscera and muscles in 

 the abdominal chamber, the tunic of the sinuses is disposed like a 

 peritoneum, seems to perform, also, the function of a peritoneum, and 

 the contained fluid, that of peritoneal serum, in addition to their own 

 more proper and important offices, and although an anatomist might 

 be permitted, by such similarity of function, to call the cavities of 

 the sinuses " intervisceral lacunae," and the walls of the sinuses "pe- 

 ritoneum," — yet, if he were guided in his nomenclature by con- 

 siderations of homology instead of analogy, he would more correctly 

 term them " abdominal venous sinuses" and " venous tunic" respec- 

 tively. In either case, as a matter of fact, there is no real or essen- 

 tial departure from a circulation in a closed system of vessels ; but 

 only a morphological departure from the typical character of the 

 organs of circulation, — an extreme one, it is true, but little likely to 

 lead astray the zootomist who had been prepared for such formal 

 modifications of the vascular system by the discoveries of Hunter, as 

 they are manifested in his preparations, and by the descriptions and 

 figures of such preparations, of the venous system in the classes 

 of Insects and Crustaceans. 



The absorbent system, as Cuvier truly states*, is wanting in 

 molluscs. In the comparatively low organised order under consider- 



* CCCXXI. p. 300. 



K K 



