20 MOLLUSC A. 



classification. The external openings are those which are 

 detected with the greatest facility, but the structure of the 

 internal organs exhibits more varied and discriminating 

 marks. 



CONDITION OF THE MOLLUSCA Molluscous animals, in 

 reference to their condition, are divided, according to the 

 situation in which they reside, into three groups, which 

 may be termed terrestrial, fluviatile, and marine. Those 

 that inhabit the land belong exclusively to the gasteropoda. 

 Among these, some prefer open pastures, others the rub- 

 bish of old walls, while not a few reside in woods or among 

 dead leaves and putrid plants. All the animals of this 

 group respire by means of a pulmonary cavity. 



The fluviatile mollusca, or such as reside in fresh waters, 

 include not only many gasteropodous genera, but likewise 

 a few belonging to the Conchifera. Amongst these, some 

 breathe air by means of a pulmonary cavity, and come to 

 the surface to respire. Such species frequent the more 

 shallow ponds and lakes. Others, respiring by means of 

 gills, are less dependent on the shallowness of the water, 

 and consequently reside in different depths. 



The marine mollusca include genera of all the classes. 

 Some burrow in the sand, of adhere to the rocks which are 

 left dry by the receding tide. These are termed littoral 

 species. Others, however, which have been denominated 

 pelagic, reside in the deep, and are seldom obtained but by 

 dredging, or when thrown ashore during storms. 



The effect of temperature in regulating the distribution 

 of molluscous animals, has not been investigated with any 

 degree of care or success. Over the terrestrial and fluvia- 

 tile species, it probably exercises a very powerful control, 



