1919] Barrows: Occurrence of a Rock-Boring Isopod 307 



invariably contain an isopod of approximately the same size as the 

 hole in which it is found. When contracted into a ball the isopod can 

 sometimes be made to roll out of its hole much as a shot may roll out 

 of a gun barrel. 



There is no other animal in evidence which could have made these 

 holes. If holes so numerous as these were freshly made by another 

 animal than the occupants found in them, one would expect to find 

 traces of the other animals alleged to have made the holes. There 

 is no indication that the animal association of these reefs contains in 

 large numbers other animals than those already mentioned in an 

 introductory paragraph of this paper. 



There are to be seen on the walls of the bores fine scratches such 

 as would be made by the mandibles or hard parts of the exoskeleton 

 of such animals as the isopod occupants in excavating the bore. 



Still stronger evidence, however, that these isopods in nature bore 

 their own holes comes from laboratory observations of their attacks 

 upon chalk. Into a glass dish containing some twenty isopods and a 

 little Viva three cone-shaped pieces of carpenters' chalk were placed, 

 two of the blocks flat side downward and the third apex downward. 

 The isopods had previously crept out of sight among the folds of the 

 Viva. Between two subsequent observations about twenty-four hours 

 apart one isopod, 4 or 5 mm. in diameter, bored for a distance a little 

 more than its own length into the base of one of the chalk blocks which 

 was flat side down. Other isopods showed an inclination to creep 

 about the base of the chalk blocks and to huddle under the inverted 

 block. Probably the instability of that block, resting on its apex and 

 curved side, prevented their treating it as a permanent abiding place. 



Within a few days the under surface of the two blocks which 

 rested with their flat sides on the bottom of the aquarium dish were 

 riddled with borings of the isopods (see pi. 17, fig. 2), some of the 

 borings striking upward into the heart of the block, with two or even 

 three isopods occasionally lodged in a single bore. The bottom of the 

 dish became covered with the fine chalk particles from the borings. 

 One bore approached very close to the side of the block so that its 

 outer wall became broken, making a sort of window through which 

 the isopod might be watched industriously at work. 



This animal did not use its feet in boring except to grip the sides 

 of the bore with the terminal claws so as to give a purchase for the 

 effective application of the mandibles. The feet are, moreover, too 

 feeble to be expected to accomplish any great effect against rock. 



