SNAILS AND LIMPETS. 



163 



are exceedingly interesting in the aquarium. The forms, 

 structures, and habits of the Univalves especially are very 

 various. For example : 



1. The shells of some snails are flat-coiled, lite the Planorb^ 

 corneus (Fig. Ill), while those of others are oval-ohlong, like 



Fig. 112. LIMN.EA STAGNALIS. 



the Lirrmcea stagnalis (Fig. 112), or ear-shaped at the opening, 

 like Limncea auricularia (Fig. 113). 



2. Some snails' shells are as much as IJin. in length, while 

 those of others are as small as a big pin's head. 



Fig. 113. LlMN^A AURICULARIA. 



3. Some snails are able to close the aperture of their shells 

 with an operculum, and others, during an enforced absence 

 from water, with an epiphragm. 



4. Some snails are either male or female; others are both 

 male and female; others again, when yoxmg, have the sexes 



M 2 



