PHYSAD^E. 7 



becomes filled, the animal generally descends.* 

 I have occasionally observed some individuals 

 to become detached from the bottom, and rise 

 suddenly to the surface;t but this generally 

 happens when they have crept down along some 

 object, with a very full supply of air. Some of 

 the species have the power to ascend and de- 

 scend, by expanding and compressing the volume 

 of air contained in the branchial cavity, a 

 method more simple than the one which Doctor 

 Buckland has demonstrated to exist in the 

 Nautiloid family. I have thus observed Physa 

 heterostropha slowly to rise through the water, 

 open its foramen, and descend again with an 

 equally slow motion.^ Without a power of 

 this kind, these animals would not be able to 

 attain the surface, in situations where they 

 might accidentally be deprived of aquatic plants, 

 or other extraneous bodies, to which they 



* On some occasions, however, it becomes necessary, in 

 addition to the withdrawal of the foot, to allow a few bub- 

 bles of air to escape, before a sufficient specific gravity is 

 attained to allow the animal to sink. 



f Loudon's Mag. Nat. Hist., 1830, vol. iii. p. 532. 



$ It has the power also of ascending and descending in 

 water a few inches deep, by the aid of a glutinous thread. 



