570 THE POPULAR SCIENCE MONTHLY. 



respiratory organs of the mussel, and this orifice corresponds to the 

 siphon in the sea-snails already studied. This current of water, be- 

 sides bathing the gills, also carries in minute particles which are float- 

 ing in the water, and these particles are conducted to the mouth of 

 the creature, and swallowed as food. At the opposite end of the shell 

 from these openings, or the forward end, a whitish, fleshy mass will 

 be seen protruding. This is called ihefoot^ and corresponds to the 

 foot or creeping disk in the snails. By means of this foot the mussel 

 crawls through the sand. 



The mouth is above the foot, and always concealed within the shell. 

 In Fig. 7 the foot is shown, and also the excurrent and incurrent ori- 

 fices, with arrows drawn to indicate the direction of the currents of 

 water. 



In some small species of fresh-water bivalves, the excurrent and 

 incurrent orifices are prolonged into tubes, and then they are called 

 si2>ho?is. Fig. 9 represents a common species which the pupils may 

 find in muddy brooks and ditches. By using the long-handled dipper 

 already described, some specimens will jjrobably be found. They are 

 quite small, from the size of a pea to that of a nickel cent. The 

 siphonal tubes are prominent, and the foot is long and tongue-shaped, 

 and the animal is very active in crawling about ; also in Fig. 4 ^ and 

 Jj represent two of these small animals with bivalve shells. 



Fig. 9. 



The foot of these creatures resembles in appearance and action the 

 foot of a fresh-water snail, only there is no mouth nor tentacles in 

 sight. These parts are present, but are never protruded beyond the 

 edges of the shell. 



When the fresh-water mussels are partly open, a fleshy border will 

 be seen just wnthin the edges of the shell, and this is the border of 

 the mantle, and corres[)onds to the same parts described in the snails ; 

 the fringed membrane which formed the openings at the hinder part 

 of the mussel is simply a continuation of the mantle. 



"When the shells are removed from the animal, the mantle will be 

 found lining the shells, just as the blank pages line the inside of a 

 book-cover. While the edge of the mantle deposits the successive 

 layers, which increase the size of the shell, the entii'e surface of the 

 mantle deposits the pearly substance which lines the inner surface of 

 the shells, and which is so characteristic of the fresh-water mussels. 



Grains of sand, or other particles, getting in between the mantle 

 and the shell, are soon covered by layers of pearly substance poured 

 out, or secreted by the mantle. In this way pearls are formed. 



