O PHYSAD^. 



monoica; and to Ferussac's order Pulmonalia 

 inoperculata, and sub-order Hygrophilidse. They 

 live in the water, but most of the genera are 

 obHged to come to the surface occasionally as 

 they breathe air. From the lightness of their 

 shells, they possess in an eminent degree the 

 power to move along in an inverted position, 

 with the foot applied to the surface of the water, 

 thus affording the observer an opportunity to 

 examine the oral organs when put into opera- 

 tion by the animal. The foot is adapted for a 

 more rapid locomotion than most of the preced- 

 ing families possess. This is particularly ob- 

 vious in Physa, which is the most active, and, 

 as I believe, the most highly organized genus 

 in the family. The members of this family are 

 generally confined to stagnant waters, or those 

 parts of small streams which are not liable to 

 be continually rippled, as this would render the 

 opening of the branchial cavity at the surface 

 a matter of some difficulty. 



The foot appears to be covered with a secre- 

 tion which has a repulsion towards water, as 

 that organ will form a concavity at the surface, 

 into which the water will not readily flow; but 

 when this cavity is so far withdrawn that it 



