or those mollusks which have a large foot on the lower surface ; 

 we next note that it can be placed in the order Pulmonata, which 

 comprises those snails which breathe by a lung. Its next position 

 is found in the suborder Basommatophora, which includes those snails 

 having flattened and contractile tentacles, with eyes placed at their 

 inner bases. These live for the most part in the water, but come 

 to the surface for fresh air. Following out our analysis, we next 

 find that it belongs to the family Limnceidce, which includes the pond 

 snails and orb shells. As it has a long spire it belongs to the genus 

 Limncea. 



"If we tabulate this, we find that it appears as follows: 



Class, Gastropoda. 

 Order, Pulmonata. 



Suborder, Basommatophora. 

 Family, Limnseidse. 

 Genus, Limnaea. 



Species, stagnalis Linne". 



"The specific name, stagnalis, signifies pond inhabiting, and indi- 

 cates that the animal lives in ponds or lakes. In a number of mol- 

 lusks, the specific name has a distinct meaning, and refers to some 

 characteristic of the animal or shell; but in the majority of cases the 

 name is simply arbitrary, and is used as a means of identification, 

 just as are the names of persons, or the trade names of merchandise." 



As the Professor ceased speaking, George inquired what the difference 

 was between a genus, a species, and a variety. " That question," 

 replied Professor Parker, "is a little difficult to answer in such a man- 

 ner as to make it perfectly clear to you. A genus includes all of the 

 animals having certain characteristics in common, as the genus Limncea, 

 which includes the pond snails with a long spire. The animals may 

 also have certain features which distinguish them from other related 

 animals, as the flat, triangular tentacles of Limncea, which are differ- 

 ent from the long, tapering tentacles of the orb snails, Planorbis. 

 Species are distinguished by many minor features, the comparative 

 length of the spire, the condition of the sutures separating the whorls, 

 whether impressed, channeled, or plain, and the presence or absence 

 of an umbilicus, or teeth within the aperture: all of these character- 

 istics and many others serve to distinguish one species from another. 

 A variety is very difficult to describe, as it is simply a minor modifica- 

 tion of a species. To use a familiar illustration, the domestic cat, 

 Felis domestica, is a species which comprises numerous varieties, as 

 a black cat, a white cat, a black and white cat, or a tortoise-shelled 



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