GASTEROPODA. 277 



tentacles ; their mode of respiration obliges them 

 to come frequently to the surface of the water to 

 breathe. They consequently 

 cannot keep at great depths, 

 and ordinarily reside in fresh 

 waters or near the mouths of / 



rivers. 



The Pond Snails (Limnteus), 

 common in every pond, live upon ^ 

 vegetables and the seeds of water- FIG. 2i3.-LiMN.Eus AURICU- 

 plants, and for this purpose are 

 provided with a strong muscular gizzard. 



The Flat Coils (Planorbis) are recognised by their 

 shells being rolled up spi- 

 rally in the same plane, 

 like a French horn. Their 

 habits are similar to those 

 of the Pond-snails, of which 

 they are the constant com- 

 panions. Their presence 

 in an aquarium is useful, 

 inasmuch as they destroy 



voraciously the green conferva^, that otherwise are 

 apt to accumulate on the sides of the glass, 



ORDER PECTINIBRANCHIATA.* 



The Comb-gilled Gasteropods (Pedinibranchiata) con- 

 stitute by far the most numerous order of the class. 

 They are so called because they breathe by means 

 of gills disposed in the form of a comb, and arranged 

 in one or two rows suspended from the interior of a 

 chamber or cavity, situated in the last- formed or 

 most capacious whorl of the shell, and communicating 

 with the surrounding element by means of a wide 

 channel or tube called the siplion. The multitudinous 

 species of marine Mollusca that inhabit spiral or 

 univalve shells belong to this order. Many of them 

 have a shelly or horny plate attached to the hinder 

 part of their body called the operculum: this .accu- 



* Pecten. a comb ; branchiae, aills fimnh-n 



