1919] Barrows: Occurrence of a Rock-Boring Isopod 309 



and strengthen the outgoing current, which emerges as a pulsating 

 stream below the posterior end of the body. 



Moll (1914) states that it has been impossible to demonstrate the 

 presence of secretions in the case of Chelura, a related amphipod 

 borer, which might aid in the process of boring, but adds that ligneus 

 chips have been found in their stomachs (i.e., of Chelura). It seems 

 certain therefore that fragments of wood gnawed off by Limnoria or 

 by wood-boring species of Sphaeroma will be found in the stomachs 

 of these borers as well, though whether this wood is made to con- 

 tribute to the nourishment of the borer is another matter. Chalk 

 particles readily identified by color, consistency, and acid reaction 

 have been found in the alimentary canal of specimens of Sphaeroma 

 pentodon which have bored into chalk, indicating a certain amount 

 of lack of selection or of accident in admitting material into the 

 digestive tract. The greater amount of chalk debris resulting from 

 the excavation is, however, washed directly out of the bore by the 

 swimmerets. 



It is not known how extensively these borers may leave their bores 

 when covered by the tide. If exclusively plankton feeders, they 

 probably rarely make excursions away from their holes. In the 

 laboratory, specimens of rock containing numerous borings with 

 isopods in the holes have been kept for weeks and only very rarely 

 have the isopods been seen to leave the holes voluntarily to wander 

 over the surface of the rock or to swim about the aquarium. 



Both in the laboratory and in the natural condition in the rocks 

 of the bay shore the isopods have been repeatedly seen resting in the 

 mouth of the bores, probably because a current of water for good 

 respiration and food supply can be the more easily set up there than 

 at the head of the bore. The animal, however, invariably presents 

 the posterior end outward. It is to be noted in this connection also 

 that the uropods seem to be exceedingly sensitive, probably acting as 

 efficient tactile organs in the place of antennae in perceiving danger. 

 When alarmed the isopods retreat to the inner end of the hole, and if 

 pursued roll up into a nearly spherical ball, so that the popular term 

 of "pill-bugs," given to the group, seems very appropriately applied. 



That the boring habit was thoroughly established in the race of 

 the isopods brought into the laboratory appeal's from the activity with 

 which the isopods attacked the chalk. If this race be morphologically 

 the same as that collected from the mudflats and marshy ground by 

 Bitter and Holmes, it seems to differ from those races physiologically 



