MOLLUSCS, VERMES AND HYDROZOA 



P. heterostropha. Fig. 176, reaches a length of shell ^ Inch and is to 

 be found on the water plants of ponds and streams quite generally in the 

 Eastern and Middle States. The elongated translucent polished spindle- 

 shaped shell is ovate, of 

 yellowish-brown color 

 terminating rather 

 abruptly in an acute apex, 

 and the large aperture is 

 oval, about three-fourths 

 the length of the shell. 



Of the four or five slight- "^- '^S- Phpa h.„ro.,roph.. Enlarged. 



ly convex whorls the body whorl is inflated and more than half the length 

 of the shell. The suture is well marked, the apex pointed and the aperture 

 narrow and longer than the spire. The body is yellowish-grey or darker, 

 dotted with whitish-yellow; and the long and slender tentacles are pointed. 

 It is an active snail but of objectionable vegetable feeding habit. It de- 

 posits 6 to 20 eggs in a capsule, which hatch in 15 to 20 days. Found 

 generally throughout the Eastern and Middle States and in the Delaware 

 and Schuylkill rivers. It is frequently introduced into the aquarium with 

 aquatic plants, and is one of the most common freshwater snails. 



Aplexa hypnorum. Fig. 177, has a slender translucent highly polished 

 yellowish-brown or ochre shell, of five or six oblique revolving flattened 



whorls with convex apex 

 and narrow aperture. The 

 body is black, which tends 

 to give the thin shell also a 

 black color. It inhabits 

 stagnant pools and is found 



FIG. 177. AfUxa hypnorum. Enlarged. \^ slow-flowing Streams of 



Vermont, Massachusetts, New York, Pennsylvania and the Western States. 



There are many other species of Physa and Aplexa, but not native 

 to the Fasten and Middle States. As they are largely herbivorous and 

 therefore objectionable in the aquarium they need not be described. 



General Remarks. Descriptions of the colors of the shells of 

 snails can only be done approximately as they are variable and depend 

 upon the character of the water which the snails inhabit. The presence of 

 iron, for instance, stain them to darker colors or black, while slight acid- 

 ities bleach them to paler colors. If the empty shells are laid in oxalic 

 acid their true colors will soon appear. Those given are based on the ob- 

 servations of the author, either of living snails in his aquaria or of those 

 in his cabinet collection. 



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237 



