590 ORGANS OF SECRETION. 



of a glutinous secretion, particles of gravel, shells, and other 

 substances, for concealment and protection. 



The liver of the conchi/era, as in the higher tunicata, con- 

 sists of numerous small lobes, compactly united together, 

 surrounding the pyloric portion of the stomach and part of 

 the intestine, and opening freely by numerous short wide 

 ducts into the capacious gastric cavity. The lobes consist of 

 racemose groups of small dilated short biliary follicles, filled 

 with their dark-coloured secretion, and opening into common 

 anastomosing ducts, which penetrate the coats of the 

 stomach by separate wide oblique orifices. In the situation 

 of the pancreatic gland of higher mollusca, there is a single 

 elongated and wide follicle opening into the stomach of many 

 conchifera which secretes the firm cartilaginous material of 

 the gastric dart, and salivary follicles are seen in some of the 

 brachiopodous species, as in the lingula, A copious secre- 

 tion of calcareous matter for the increase of the shell, is 

 thrown out from the vascular exterior of the mantle, and a 

 mucous secretion from its interior. In the abdomen, below 

 the heart, is a considerable reticulate soft glandular organ 

 and sac with two narrow orifices, near those of the oviducts, 

 generally charged with calcareous particles and uric acid, 

 and regarded by Swammerdam as the calcifient gland, by 

 Blainville as a urinary apparatus, and by Bojanus as respi- 

 ratory. Many genera, especially those provided with thin 

 delicate shells, are fixed by a byssus, composed of strong 

 dense filaments, secreted by a distinct gland at the base of 

 the foot, and these filaments appear to be intimately united 

 to the muscular or tendinous fibres reaching from the foot 

 to the shell. The pearls formed by these animals are com- 

 posed of spherical concentric layers of nacreous matter, 

 secreted around foreign particles by the outer surface of 

 the mantle, or in some by appropriate calcifient glands, and 

 the same parts afford the various colouring matters which 

 decorate the shells. 



The glandular organs of the gasteropods are more numerous 

 and distinct, and correspond with their more varied and 

 active life. The hardness of their food now necessitates 

 masticating apparatus, and these necessitate salivary glands, 

 which keep pace with them in development. The salivary 



