254 FUNCTIONS AND INSTINCTS. 



Many other bivalves, which I need not particu- 

 larize, spin a byssus with their foot. Singular it is 

 that the same office should be assigned to organs 

 so differently situated in different animals. The 

 spinnerets of the silk-worm, and other spinning 

 moths are in the mouth, those of the spider in its 

 tail, and those of various shell-fish in their fool; 

 in the first case, if we consider the various pur- 

 poses to which caterpillars apply the faculty of 

 spinning, we see the importance of its being 

 under the direction of the eye of the animal : 

 and even in the case of the spider, the eye 

 directs the animal in its course to form its con- 

 centric circles, and the thread follows it ; and the 

 same is the case when it spins the rays that 

 traverse its web ; and when it descends from a 

 height the same takes place. But the foot is 

 the only organ that is so situated in bivalve 

 shells, as to throw forth a thread that will go 

 out of the shell, where it is wanted for use. 



Of all this tribe of shells none are more beau- 

 tiful, both as to their form, painting, and sculp- 

 ture, than what are called Escallop shells, or Comb 

 shells 1 from their resemblance, as to the scoring 

 of the upper valve, to that instrument. These 

 may be regarded as, in some degree, analogues 

 of the butterflies amongst insects, and their fly- 

 ing as it were, on the surface of the water, as 



1 Pecten. 



