106 ORGANS OP SECRETION. 



heart The liver embraces the stomach and a portion of the 

 intestine, its acini forming numerous lobules which open by 

 orifices into the former, and the anterior portion of the latter. 

 There is usually a blind sac ; sometimes, as in Pholas, Tellina, 

 etc., verj' long, and extending between the circumvolutions of 

 the intestine. Within this is generall}^ lodged a transpa.rent, 

 cylindrical, cartilaginous body called the crystalline stylet, which 

 is sometimes present, sometimes absent in the same species, 

 from which Siebold infers that its appearance is periodical. It 

 is probably an accessory digestive organ ; but appears to be 

 absent in some of the Monomyaria. In the TJnionidas the 

 stylet is found in the intestine (it is over half an inch long- 

 in U. cariosus), and the blind sac is not developed. Those 

 Pelecypoda having a closed mantle receive through the incurrent 

 siphonal tube such microscopic food as maj^ accompany the 

 water inhaled for breathing ; in others the water has free access 

 to the gills ; and in both cases the ciliated lamina of these organs 

 arrest floating particles of food and mould them into threads by 

 a viscid secretion of the surface ; they are then propelled in the 

 direction of the mouth, which thej enter between the palpi. 

 Minute vegetable organisms and animalcules are found in great 

 abundance and variety in the stomachs of bivalve shell-fish. 



The renal organs are paired, and have received, after the name 

 of their discoverer, the appellation of Organs of Bojanus. They 

 consist of symmetrical glands, at the base of the branchiae, in 

 the dorsal region. The excretory canals of these glands open, 

 in Mactra, Unio, etc., contiguous to that of the genital organs; 

 in Area and Pinna they unite with the oviducts ; in the Pectinidse 

 the genital glands pour their products directly into the Organs 

 of Bojanus. In the Unionidse, Cardium, Pholas, etc., Lacaze- 

 Duthiers has found another orifice by which this organ commu- 

 nicates with the pericardium. The organs of Bojanus are 

 brownish or greenish yellow, spongy, and secrete uric acid, with 

 concretions of carbonate of lime. 



ORGANS or SECRETION. 



Besides the salivary and urinary organs described in connec- 

 tion with the chapter on the digestive s^^stem, there exist in 

 some mollusks special organs, the products of which are suffi- 

 ciently important in character to require particular description. 

 The first of these is the Ink-gland of the Cephalopoda. 



The ink-bag or anal gland (ix, 93), (not present in Nautilus), 

 is a tough and fibrous sack, the outer coat of which is thin and 

 silvery ; the contents are discharged by a duct direct ( or, as in 

 most decapods, through the anus) into the mantle opening, and 

 thence ditt'used in the surrounding water ; coveinng the move- 

 ments of the animal by the obscurity in which it becomes almost 



