No. 521] FACTS ABOUT THE « LOBSTER PEARL " 299 



arthropod, since the hard shell is a differentiated por- 

 tion of the outer ends of the epithelial cells themselves 

 and in direct organic connection with them, except un- 

 der the peculiar conditions which determine the molt. 

 The shell of a pearl- secreting mollusk, on the other 

 hand, is a true secretion product, to which deposits are 

 successively made by epithelial cells having a different 

 relation to the shell, which is never cast off. Assuming 

 that a foreign body could by any means find lodgment 

 between the hard and soft parts of a lobster's skin, it 

 would not be free to move as is the case with a grain 

 of sand inserted beneath the mantle of a bivalve mol- 

 lusk, and if it did not immediately set up a process of 

 regeneration at the point of lesion, the foreign body 

 would be surely lost at the succeeding molt. 



If the shell of the claw to which this spherical body 

 pertained had been preserved, its origin could have been 

 traced with greater certainty, but all things considered, 

 it seems to be a vagary of the process of regeneration, 

 due in all probability to some peculiar injury, leading 

 to an ingrowth or pocketing of the skin at that point, 

 instead of to the usual protuberance. It represents a 

 permanent ingrowth of a part of the shell, started in 

 all probability when this was soft, and not later 

 smoothed out or effaced at any subsequent molt. While 

 it would be impossible to prove that this body was not 

 formed, as we see it, during the interval between two 

 molts, it seems quite possible that it has survived more 

 than one casting of the shell, in which case we should 

 have a succession of "pearls," similar to this one, but 

 probably forming a progressive series as regards their 

 size and solidity. 



A cruder suggestion would be that this sphere repre- 

 sents one of the grinding tubercles or " molar teeth" 

 of the crusher claw, like the large proximal and usually 

 symmetrical tubercle of the dactyle, pressed into the 

 meat of the claw in some unaccountable manner, and not 

 later restored to a normal condition. The presence of 



