MOLLUSC A. 43 



their means, not only how to work in the schoolroom, 

 but how to collect and observe out of doors. 



These, in many species, have very thick shells, whose 

 structure can be studied with great ease ; but the beaks 

 of the shells are almost invariably wanting. The acids 

 contained in fresh water seems to act upon these, which 

 are always exposed above the mud in which the animal 

 lives, eating them away until in some specimens they 

 are almost wholly removed. 



The sexes, also, are very distinctly marked, since the 

 females are usually stouter than the males, and have 

 larger gills, especially during certain seasons, when 

 these are filled with the young fry, which are stored 

 away in their interior until they are large enough to 

 shift for themselves. 



SNAILS. 



The Pteropods, the Butterflies of the Sea, and many 

 other curious and strange forms, such as the Hetero- 

 pods, might be described in these pages ; but the im- 

 mediate object of the lesson would not be furthered 

 by these digressions, and besides, they are fully treated 

 of in many other books. 



It may be well, however, to say that in one division 

 they have shells, and are headless, with broad wings 

 for swimming, — these wings being probably part of the 

 foot, of which also a rudiment is left. Fig. 39 gives 

 an idea of Hyalea, one of these forms. The other 

 division is composed of Pteropods having a swollen 

 extremity like a head, and with a mouth surrounded 

 by tentacles. Figs. 38, 34 show Clio, commonly known 

 also as " whale's food." They are all free, swim- 



