Molluscs 181 



thus : Unios and their allies are large forms that have 

 pearly shells and that live mainly in large streams and 

 lake borders. They produce enormous numbers of 

 young, and use mostly the outer gill for a brood 

 chamber. They cast the young forth while still minute 

 as glochidia, to become attached to and temporarily 

 parasitic on fishes. The relations of these larval 

 glochidia with the fishes will be discussed in chapter V. 

 The lesser mussels (family Sphaeridae) dwell in small 

 streams and pools and in the deeper waters of lakes. 

 Their shells are not pearly. They produce but a few 

 young at a time and carry these until of large size, 

 using the inner gill for a brood-pouch. The stouter 

 species, half an inch long when grown, burrow in stream- 

 beds like the unios. The slenderer species climb up 

 the stems of plants by means of their excessively mobile 

 adhesive and flexible foot. On this foot the dainty 

 white mussel glides like a snail or a flatworm, up or 

 down, wherever it chooses. 



Snails are as a rule more in evidence than are mussels, 

 for they come out more in the open. They clamber 

 on plants and over every sort of solid support. They 

 hang suspended from the surface film, or descend there- 

 from on strings of secreted mucus. They traverse 

 the bottom ooze. We overturn a floating board and 

 find dozens of them clinging to it, and often we find 

 a filmy green mass of floating algae thickly dotted with 

 their black shells. 



They eat mainly the soft tissues of plants, and micro- 

 organisms in the ooze covering plant stems. A ribbon- 

 like rasp (radula) within the mouth drawn back and 

 forth across the plant tissue scrapes it and comminutes 

 it for swallowing. Because snails wander constantly 

 and feed superficially without, as a rule, greatly altering 

 the form and appearance of the larger plants on which 



