THE LAMELLIBRANCHIATA. 411 



to a large median venous sinus; from this it passes through the 

 walls of the renal organs to the gills, and is returned from 

 these to the auricular division of the heart. 1 Very generally 

 the ventricle invests the rectum, but in Ostrcea, Teredo, and 

 A?iomia, the ventricle is quite detached from the intestine. 



The renal organs, or organs of Bojanns, are usually two 

 in number, often more or less united together, of a dark color, 

 situated beneath and behind the pericardium and in front 

 of the posterior adductor muscle, extending forward on each 

 side of the mesosoma, and traversed by such numerous blood- 

 channels, that they have a spongy texture. The walls of the 

 cavernous blood-sinuses are lined with cells which secrete the 

 urinary matters from the blood. These take the form of cal- 

 careous concretions, containing uric acid. The gland commu- 

 nicates at one extremity with the pericardium ; at the other, 

 it either opens directly on to the surface of the body, or into 

 a vestibular cavity which has an external aperture. 



In Ostrcea and Teredo the renal organ seems to be present 

 in only a very rudimentary form. 2 



The mesodermal region, between the endoderm and the 

 ectoderm, is for the most part occupied by vascular, connec- 

 tive, and muscular tissues, and by the reproductive organs, 

 so that there is no large perivisceral space. But there is — 

 1. The large median sinus already mentioned, which receives 

 the blood returned from all parts of the body, and is com- 

 monly termed the vena cava. 2. A spacious pericardial 

 chamber which incloses the heart. It is in communication 

 with the venous system, and, consequently, directly or in- 

 directly, with the vena cava. 3. The cavities of the renal 

 organs, which usually freely communicate with one another, 

 while they open into the pericardium on the one hand, and on 

 the exterior of the body on the other. 4. In some Lamelli- 

 brancliiata, canals open on the exterior of the body, especially 

 on the surface of the foot. In this way the blood-system is 

 placed in direct, though circuitous, communication with the 

 surrounding water. These so-called water-vessels com muni- 

 cate internally with the venous system, of which, indeed, 

 they seem to form a part. It is probable that all these cavi- 

 ties, taken together, represent the perivisceral cavity, pallial 

 sinuses, and pseudo-hearts of a Brachiopod. 



i The circulatory orerans of the fresh-water Mussel have been very fully de- 

 scribed by Langer. (" Denkschriften der Wiss. Akademie, " 1855 and 1856.) 



a See, for the structure of the renal organs and many other points connected 

 with the anatomy of the LamellibraneMdta, the series of valuable papers of La- 

 caze-Duthiers. (" Annales des Sciences JNaturelles," 1854 to 1861.) 



