264 Introduction to Animal Morphology. 



motion of the gills of marine bivalves is immediately 

 checked by immersion in fresh water. 



The organ of Bo j anus is yellow or brown, spongy, 

 bilaterally symmetrical, placed dorsally next the base 

 of the gills, the lateral parts being sometimes medially 

 united. 



It is tubular, opening within into the pericardium by a 

 radially lamellar extremity which surrounds the tendon of the 

 posterior retractor, and lies on the front and lower surface of 

 the hinder adductor; its central part is sometimes dilated 

 into a spongy cavity ; the outer part opens on each side into 

 the mantle cavity. Into its spongy tissue the blood from the 

 viscera enters by a portal system of vessels, and mixes with 

 water, admitted by the duct of Bojanus. In its recesses in 

 Lutraria and Mactra uric acid is found, and, in Pectunculus 

 pilosus, granules of calcium phosphate. In Teredo this or- 

 gan is a dark layer, containing urate of ammonia, overlying 

 the auricle of the la-art ; in Ostrea it is also appended to the 

 auricle close to the commissure between tin- epipharyngeal 

 and visceral ganglia, and contains concretions. The sex or- 

 gan opens into this organ in Pecten, Lima, and Spondylus, or 

 the duct of the sex organ unites with its duct in Area, Modi- 

 ola, and Mytilus ; in Pinna they open on the same papilla, 

 but the sex duct is the farther back. The ducts are separate 

 in Cardium, Chama, Pecten, Unio, &c. This is a modified 

 segmental organ ; its cavity is regarded by Rollcston as a 

 water-vascular organ ; its function is excretory, probably uri- 

 nary. 



The nervous system consists of ist, an epipharyn- 

 geal (labial, cerebral) ganglion on each side, either re- 

 mote and with a long commissure (Lucina, Area, 

 Mytilus, &c.), or close together (Venus, Mactra, &c.) ; 

 or united (Teredo) ; this lies on the tendon of the re- 

 tractor pedis anterior, and in front of the protractor 

 pedis, above the mouth, and gives off on each side 





