MOLLUSCA. 415 



tected with the greatest facility, but the structure of the in- 

 ternal organs exhibits more varied and discriminating marks. 



Condition of the Mollusca. Molluscous animals, in re- 

 ference to their condition, are divided, according to the si- 

 tuation 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 

 rubbish 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. Among 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, or 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 controul, 

 greatly limiting their geographical range. In proof of this, 

 it may be stated, that the south of France possesses several 

 species not to be found in England, while in England, 



