in much the same manner as is done by the land shells. This fact 

 accounts for the apparent disappearance of all life from a pond when 

 it dries up, and its sudden and seemingly unaccountable reappearance 

 when the pond is again filled with water. 



" Next to the Limnceas, the Planorbes, or orb-shells, are the most 

 abundant and interesting, although not exhibiting a large amount 

 of variation, as you will see by the specimens in this case. Their 

 rounded, orb-like shells are found along the shore of almost any pond 

 or stream. In size the shells vary from the little Planorbis parmis, 

 so common among the fresh-water algce and which is scarcely an 

 eighth of an inch in length, to the giant Planorbis corneus of Europe, 

 with a shell over an inch in diameter. The Planorbes are found in 

 nearly all parts of the world. 



"The family Physidce, containing left-handed or sinistral shells, 

 is one of the most distinct of all the fresh-water mollusks, not only 

 in the form of the shell, but also in the character of the animal, 

 the mantle being ornamented by many little finger -like projections 

 w T hich are reflected over the inner lip of the shell. We learned some- 

 thing about these animals on our last collecting trip; how they move 

 over the bottom of a pond or creek with a steady, gliding motion, 

 even when the surface of the water may be frozen. The shells are 

 almost always smooth and glossy, and average from half an inch 

 to an inch in length. Like the Limnceas and Planorbes they are 

 found in many parts of the world. The egg-masses of Physa, Pla- 

 norbis, and Limncea (little, glairy, transparent, jelly-like objects) may 

 be seen in the spring in almost any pond or stream, attached to sticks, 

 stones, or the under side of water-plants. 



" Not all of the fresh-water pulrnonates have spiral shells. A whole 

 family, the Ancylidce, have a conical shell formed like a rounded shield 

 and resembling the limpet of the seashore; hence they are called 

 river limpets. They are generally quite small, some of the species 

 being less than a quarter of an inch in length. They live attached 

 to the interior of dead river shells, and to submerged plants and rocks. 

 They are very interesting, but hard to find on account of their small 

 size and inconspicuous habitat. 



" The second class of mollusks of which I spoke a short time ago 

 (those which breathe air through the water) have a respiratory cavity 

 instead of a lung, in which is placed a series of leaflets arranged like 

 the teeth of a comb in two series of lines, forming the so-called gills. 

 The mouth is placed at the end of a long rostrum or proboscis and 

 not in the lower plane of the head, as in the last class. We must 



19 



