32 FIRST BOOK OF ZOOLOGY. 



rent and incurrent orifices are prolonged into tubes, and then 

 they are called siphons. Fig. 37 represents a common spe- 

 cies which the pupils may find in muddy brooks and ditches. 

 By using the long-handled dipper already described, some 

 specimens will probably 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. 

 14 K and Z represent two of these small animals with bi- 

 valve shells. 



FIG. 37. 



30. 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 within the edges of the shell, and 

 this is the border of the mantle, and corresponds 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 man- 

 tle deposits the successive layers, which increase the size of 



