ii8 



THE BROOK SXAIL. 



Physa 



received special names. They are, however, essen- 

 tially alike, and may all be included 

 under the name Helisoma trivolvis^ 

 Say, tri-vol'-vis, the general form of 

 which is shown in Fig. io6. The shell 

 tube is wound in a coil, consisting of 

 Fig. io6. about four w^horls. The color is light 

 brown, and the aperture is irregular. The diameter 

 of the coil is from one-fourth to three fourths of an 

 inch. Some of the names of the varieties are given 

 for the convenience of those making exchanges. 

 They are corpiilentiis^ fragilis^ occidentalism fallax^ 

 tnmens and Oregonensis. 



In the brook which runs past Mills College I have 

 caugfht many specimens of the little Water-snail, 

 heterostropha^ Say, Fi'-sa het-e-ros'-tro-pha, 

 shown in Fig. 107. It is common in brooks 

 and streams over much of the Pacific Slope, 

 as well as east of the Rocky mountains. The 

 shell is thin and delicate, of a light horn 

 color, with a small spire, a sinistral ( left- 

 Fig. 107. handed ) aperture, and commonly is about 

 half an inch in length. The animal is black or 

 nearly so, and when the shell is inhabited it appears 

 much darker than when it is empty. 



It is amusing as well as instructive to put some of 

 these little creatures in a jar of w^ater and watch their 

 movements. Sometimes they will quietly remain at the 

 bottom, eating the pulp of an alder leaf which you 

 have given to them; then they will rise to the surface 

 to take a breath of fresh air and slowly sink back 

 again, or perhaps they will crawl along, shell down- 

 ward, apparently clinging with their foot to the sur- 

 face of the water — an apparently impossible feat, but 

 they do it nevertheless. 



