No. 628] ONCIIIDIUM AND ADAPTIVE COLORATION 421 



thysanurans so common on the beach sands and also on 

 the rocks where Onchidium lives. 



Typical Onchidia of the lighter colored variety appear 

 at all times light olive or greenish yellow. The colora- 

 tion is not ''solid," for the dark-hued internal organs 

 usually show through as one or more dull blackish 

 patches. The edge of the mantle is quite pale. 



It has not been possible to convert one variety into the 

 other by laboratory experimentation. The dark Onchidia 

 are still dark after six weeks complete starvation. 



Onchidia of either type are found, frequently side by 

 side, in all the varieties of habitat which we have de- 

 scribed. To mention some specific localities, we may 

 list the following: Bailey's Bay; Flatt's Inlet; the east 

 side of the channel entrance to Hungry Bay, at its inner 

 end; an island at the entrance to Fairyland Creek; a 

 small cove on Burgess Point. These situations are 

 widely separated, and cover the ditforent sorts of back- 

 grounds to which allusion has been made. 



Not only in a general way, merely, but also in any 

 given place where they occur, the lightly pigmented On- 

 chidia fail to indicate by any peculiarity of behavior that 

 they differ essentially from the dark ones. That they 

 are easily overlooked is quite true ; they are liomochrom- 

 ically colored in practically all the instances that have 

 been observed. But it should be noted that in one such 

 place the homochromically colored individuals were 

 found in the proportion of but one to nearly 190 of the 

 dark black ones. This was on a greenish rock surface of 

 northern exposure where a color-changing form like the 

 isopod Ligia was pale-greenish, like the pale Onchidia. 

 In other places more or less isolated colonies of fifty or 



th.- n.loration wa> .1<-(m.1.m11 y o.,vcnl<l.. rntlirr than oliva- 



IV. The munvvou< <h.rsal i-arnll:.' n!' Otnlnrlinw were 

 said by Semper (1877, 1881) to iii.licatr tho h>eation of 

 mantle-eyes; since, however, ii<> si'1im)1\ nn-vo-terminals 



