MOLLUSCA. 113 



the older part of the shell, as commonly occurs in the region of 

 the umbo, cannot be renewed. 



Pearls are formed by the deposition of matter similar to that making 

 up the pearly-layer around grains of sand, &c., that get into the shell. 

 The pearl-fisheries of Britain were once famous, the pearls being obtained 

 from a species of Unio. In China, small metal images of Buddha are 

 placed within the shells of fresh-water mussels, and thus receive a pearly- 

 coating. 



The mantle is covered externally and internally by a layer of 

 simple columnar epithelium, which on the outer side contains 

 many glandular cells, and on the inside is ciliated. The sub- 

 stance of the mantle between these layers of epithelium is made 

 of connective tissue, traversed by muscle-fibres, blood-channels, 

 and a complex network of nerves. 



The skin covering the visceral mass and foot is very glandular 

 and closely connected with underlying layers of muscle. The 

 labial palps are, like the inside of the mantle, covered by ciliated 

 epithelium. 



The shell consists of an organic basis impregnated (except in 

 the periostracum) with carbonate of lime. The prismatic layer 

 is made up of polygonal prisms packed closely together, and 

 arranged obliquely to the surface. The pearly-layer is composed 

 of numerous thin laminae, the edges of which constitute a series 

 of delicate ridges which by diffraction produce an iridescent 

 appearance. 



3. The digestive organs consist of a convoluted tube running 

 from mouth to anus and of a large digestive gland (Fig. 32). 

 The mouth (0), on each side of which are the transversely striated 

 labial palps (MS), leads into a very short gullet (oesophagus), 

 which runs almost directly upwards, just behind the anterior 

 adductor, and passes into a dilated stomach (M). This is succeeded 

 by a narrow thin-walled intestine (D), which coils about (mostly 

 in the median vertical plane) within the visceral mass, where it 

 is closely surrounded by the genital gland (G), finally curving 

 upwards and running back dorsally as the rectum through the 

 ventricle of the heart, and over the posterior adductor to terminate 

 in the anus. The cavity of the stomach is partially subdivided 

 by irregular folds, and the ventral wall of the rectum is longi- 

 tudinally infolded to form a typhlosole. A lobed digestive gland 

 (L), dark brown in colour and made up of numerous branched 

 2 8 



