the surface of the water, with little wave-like motions passing over it, 

 with shell downward and spreading foot turned upward. The snails 

 had also extended their breathing orifices to the surface of the water in 

 the form of tubes or siphons. Professor Parker remarked that the 

 animals were busily engaged in taking in a supply of fresh air. He said 

 that pond snails, orb snails, and some other mollusks breathe air by 

 means of a lung in the same manner as the land snails, and they are 

 compelled to come to the surface at intervals for the purpose of exhal- 

 ing the poisoned air and taking in a supply of pure air. 



A short distance from the place where the Limnceas were found, 

 were noticed a number of snails in the water, gliding along, with 

 a peculiar wheel-like shell set edgewise on their backs. These were 

 the Planorbes, or orb shells. The animal was much like that of the 

 pond snail, except that the tentacles were 

 very long and thin, instead of being flat 

 and triangular. The shell was tightly wound 

 like a watch-spring, and the upper and lower 

 surfaces, that is, the spire and umbilicus, 

 were in the same plane, and the full num- 

 ber of whorls (four) could be plainly seen. 

 The aperture was rounded and the outer 

 lip thin and sharp. 



Crossing over a little neck of land which extended into the waiter, 

 our party reached the small creek which flowed into the river in which 

 lived a great number of pond snails, orb snails, and another variety very 

 numerous in specimens which were gliding swiftly about as though bent 

 upon important business. "These," Professor Parker said, "are the 

 Physas." 



As these were fine, large specimens, we all stopped to gather 

 a number for our collections. The shells of these snails were about 

 three-quarters of an inch in length, highly polished, and with a short 

 spire and very large aperture. But the aperture of the Pliysa seemed 

 different from that of the pond snail. Upon comparing the two shells, the 

 Physas were found to have the aperture on the 

 left side, while the pond snails had it on the 

 right side. Professor Parker explained that 

 these two snails were typical of all gastropod 

 shells. In the great majority of snails the aper- 

 ture is on the right side, and these are called 

 dextral or right-handed shells. In a few, like 

 Pliysa, the aperture is always on the left side, 



Orb snail, Planorbis ; shell with 

 animal creeping over the ground. 

 (Binney.) 



Left-handed or sinistral pond 

 snails, Physa gyrina. 



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