MOLLUSC A GASTEROPODA. 227 



mur, to whom we are indebted for these inte- 

 resting observations, states also that the ma- 

 rine muscles possess the art of forming these 

 threads from the earliest periods of their ex- 

 istence ; for he saw them practising it, when the 

 shells in which they were inclosed were not 

 larger than a millet seed.* In Sicily, and other 

 parts of the Mediterranean, these threads have 

 been manufactured into gloves, and other arti- 

 cles, which resemble silk. 



3. Gasteropoda. 



THE Mollusca which inhabit univalve or turbin- 

 ated shells, belong to the order of Gasteropoda, 

 and have a more highly developed organization 

 than the Acephala, The part which performs 

 the office of a foot is a broad expansion of 

 fleshy substance, occupying nearly the whole 

 under surface of the animal, and forming a flat 

 disk, capable of being applied to the plane 



along which it moves. 



This is seen in the 



Planorbis (Fig. 1 05, D). 



In some species it is 

 f~ fashioned into a pro- 



* Memoires de 1'Academie des Sciences : 1711, p. 118 to 123. 

 Poll conceived that these threads are dried muscular fibres ; an 

 opinion which has been adopted by Blainville. 



