1919] Barrows: Occurrence of a Bock-Boring Isopod 301 



vegetable feeder (Bate and Westwood, vol. 2, p. 404). The related 

 wood-boring genus, Limnoria, is also a plant feeder (Zirwas, 1910, 

 p. 43). It is yet to be demonstrated, however, that Limnoria makes 

 direct use of wood fibers as food. Undoubtedly the boring species of 

 SphaA roma must live upon the microscopic organisms washed into their 

 holes by the currents of water set up by the swimmerets. 



Among the isopods the Sphaeromidae are especially noted for 

 hiding away in crevices, under rocks, and in the abandoned holes of 

 other animals. Certain species are found regularly in the empty 

 shells of barnacles. The known occurrence of Sphaeromidae such as 

 Sphaermna quadridentatum Say "under raised bark and in deserted 

 holes of Teredo, etc., of such dead trees as are periodically immersed" 

 (Henger, 1878, p. 370) in several localities along the Atlantic coast 

 is suggestive of the origin of the boring habit in the few species of 

 this genus which have adopted this manner of life. In seeking refuge 

 in all sorts of holes and crevices a few species of this genus, quite 

 accidentally at first, may have brought their stout mandibles to play 

 in enlarging or reshaping their borrowed domiciles. From this be- 

 ginning the capacity for completely excavating their own holes in any 

 soft material, either wood or stone, may have readily developed. 



One cause initiating the instinct of hiding under stones and in 

 crevices, which is common to this group, and which has perhaps de- 

 veloped later into the boring habit, may have been the effort on the 

 part of these isopods to escape from their predaceous enemies, especi- 

 ally fish. It seems hardly probable that the isopods found on the 

 shore of San Pablo Bay resorted to the boring habit to escape the 

 force of the waves. On the open coast this motive seems to drive 

 many animals to seek shelter in natural crevices and hollows or in 

 holes of their own making. But large waves seldom occur on so small 

 a body of water as San Pablo Bay. 



The development of the boring habit in the genus Sphaeroma is 

 the more remarkable in view of the very slight morphological modifi- 

 cations, if any at all, for such a habit. In fact, the specific differences 

 within the group are apparently of minor consideration in the ecology 

 of these animals, consisting of such characters as the number of 

 tubercles on the abdomen, the number of setae on the exterior margin 

 of the uropods, slight modifications of the appendages, etc. The stout 

 mandibles with which the whole group of isopods has been supplied, 

 and which seem to have been originally used in attacking gross food 

 material, have in the plankton feeding isopod borers merely been 



