56 PRACTICAL ZOOLOGY. 



in a watch-crystal or on a slide. Examine with a one- 

 inch objective or a lens. Make a drawing of the young 

 clams. Compare the shapes of the young and adult. 

 Watch carefully for movements of the young clams. 



The shell grows by additions deposited by the mantle 

 on the inner surface of the valves. Each new layer 

 projects beyond the others; so the shell grows in width 

 as well as thickness. Where is the thickest part of the 

 shell? If a thin piece of glass is slipped between the 

 mantle and its corresponding valve, and the clam be kept 

 alive, the shell substance is deposited over the glass, and 

 the glass becomes buried in the nacreous matter of the 

 internal layer. Grains of sand become pearls by the 

 secretion of similar nacre around them. 



Read "The Fresh-water Mussel" in Huxley and Mar- 

 tin's " Practical Biology " ; "The General Anatomy of a 

 Lamellibranch" in Brooks' "Handbook of Invertebrate 

 Zoology " ; No. VI of Hyatt's " Guides for Science Teach- 

 ing " ; Packard's " Zoology." 



THE SNAIL. 



A dipper with a perforated bottom, attached to a 

 wooden handle, will be found convenient in scooping 

 up the sand and mud from the bottoms of ditches and 

 streams; the dirt being sifted out, the shells and other 

 objects will be left behind. Get a number of live snails, 

 and keep them in a fruit-jar. 



1. The broad disk on which the snail creeps is the 

 foot. 



