502 PHILOSOPHY OF 'ZOOLOGY. 



are in part united with the cloak, while in others they are 

 free to the base. In their structure they resemble the 

 branchiae. The stomach is full of cells, the bottom of each 

 pierced with a biliary duct. A singular organ, termed the 

 crystalline process, cylindrical, cartilaginous, and transpa- 

 rent, is found in some species projecting into the cavity of 

 the stomach. The liver is large, surrounds the stomach, 

 and pours out its contents by numerous openings. The in- 

 testine terminates posteriorly by a tubular anus. 



The branchiae consists of two ribbands on each side, ex- 

 tending the length of the body, free on the sides and mar- 

 gin, and striated transversely. These plates are frequently 

 of unequal size. The blood is brought to these by means 

 of pulmonic veins, without the intervention of the heart. 

 The aerated blood is transmitted to a systemic heart, con- 

 sisting of one or two auricles, and a ventricle. 



The most important of the peculiar secretions of the 

 animals of this class is the Pearl. This substance, equal- 

 ly prized by the savage and the citizen, is composed, 

 like shells, of carbonate of lime, united with a small por- 

 tion of animal matter. Pearls appear to be exclusively 

 the production of the bivalve testacea. Among these, all 

 the shells having a mother-of-pearl inside, produce them oc- 

 casionally. But there are a few species which yield them in 

 the greatest plenty, and of the finest colour. The most re- 

 markable of these is the Avicula margaritifera. This shell, 

 which was placed by LINN^US among the mussels, is very 

 widely distributed in the Indian Seas ; and it is from it and 

 another species of the same genus, termed Avicula liirundo, 

 found in the European seas, that the pearls of commerce 

 are procured. The Pinna, so famous for furnishing a bys- 

 sus or kind of thread, with which garments can be manu- 

 factured, likewise produces pearls of considerable size. 



