UNIVALVE MOLLUSC ANS. 281 



produce those spines, 8cc. and is withdrawn 

 when they have acquired consistence enough 

 not to bend when thus left to themselves. Other 

 conchologists, particularly one of the most emi- 

 nent of our times, Poli, think that the shells of 

 univalves are organized bodies, and produce 

 their spines as vegetables do their prickles, he 

 says also that their shells contain cellular mem- 

 branes almost like a Rete mucosum. 



In the progress of a shell's growth, as new spines 

 are formed old ones drop off, how this is effected 

 seems not to be accounted for by either hypo- 

 thesis it is analogous, however, in a great de- 

 gree, to what was mentioned above with regard 

 to the holes in the shell of the sea-ear, only that 

 with them an old hole is stopped up, when a 

 new one is formed. All that can be said on the 

 subject is that the animal, instructed by Provi- 

 dence, as new processes are formed and a new 

 whirl of its shell completed, is enabled to throw 

 off by a solvent, or some other means unascer- 

 tained, those that are no longer wanted. 



It is observable that the terrestrial univalves, 1 

 of this Order, are never armed with spines, tuber- 

 cles, or other elevations, but exhibit generally a 

 levigated shell. As they move about usually 

 amongst bushes, under moss, or in grass, the 

 object of the Creator in this structure was pro- 



1 Helix, &c. 



