XIIl] ANODONTA 299 



whence the name Pelecypoda (Gr. 7re'A.e/d;s, a hatchet), and is used 

 as a plough to force a way through the mud in which the creatures 

 live. There are many species of pond- or river-mussels in North 

 America : Anodonta cygnaea is perhaps the commonest in England, 

 but in places Unio pictorum is abundant; A. cygnaea occurs in 

 Canada and the United States and in these countries Unio com- 

 planatus is also common. Any one of these forms will serve our 

 purpose. The shell is about four inches long and two inches high, 

 and is covered with a black horny layer, the so-called 

 periostracum. The shell is apt in places to be 

 eroded by the action of the carbonic acid in the water. Under- 

 neath it is a thick slightly translucent layer of crystals of carbonate 

 of lime, called the prismatic layer. The inner part next the 

 mantle is composed of thin layers placed one above the other. 

 This is the mother-of-pearl or nacreous layer, which in many 

 Molluscs has an iridescent sheen, owing to its action on light. 

 These three layers are also present in the shell of the snail and in 

 all other Molluscan shells, but they are very easily made out in the 

 shell of the pond-mussel. To the periostracum the colour of the 

 Molluscan shell is mainly due. The periostracum and prismatic 

 layers are formed by the edge of the mantle and if destroyed they 

 cannot be replaced. The nacreous layer is deposited by the whole 

 surface of the mantle. If by chance a grain of sand gets wedged in 

 between the mantle and the shell it is apt to become covered with 

 layers of mother-of-pearl, and in this way a pearl-like blister is 

 formed. The more costly pearls however arise within the soft parts 

 of the body, and appear to consist of concentric layers of nacreous 

 substance surrounding some parasitic larva and secreted by the 

 mollusc as a defence against the attacks of the parasite. The shell 

 is marked by a series of curved lines running parallel to one another. 

 These lines mark the limits of growth attained in each year, the 

 amount intervening between two lines being the amount of growth 

 accomplished in a year. It will be seen that the common focus 

 around which the curves run is not in the centre of the hinge line, 

 but decidedly nearer one the anterior end. This common focus 

 is called the umbo, and it represents the shell with which the Unio 

 started life (6, Fig. 136). 



As might be expected from the shape of the shell, the mantle 

 has the form of two great flaps hanging down at the sides of the 

 body. The flaps have a free edge in front, below and behind, but 

 pass into the general wall of the body, with which they fuse, above. 





