PREFACE. 



After several 3 7 ears spent in the preparation of the present mono- 

 graph, I now offer it to the scientific world, hoping it will meet with 

 favorable consideration. Most of the work has been done in the 

 United States National Museum, where an opportunity was afforded 

 for carrying on the investigation through access to the rich collections 

 and use of the library. The summer of 19(M was spent at Woods 

 Hole, Massachusetts, and occupying a table in the Marine Biological 

 Laboratory through the courtesy of the Carnegie Institution, I con- 

 tinued the studies which I had already begun in Washington. 



It will be noticed that but few changes have been made in the clas- 

 sification. In a group where there are so many good workers, I have 

 been glad to follow and adapt to my use what has been already done 

 so well, and keys and synopses of genera have been freely introduced 

 with references to those authors from which they have been taken. 



Although I am indebted to many friends for their kind assistance, 

 my obligations to Dr. Theodore Gill are especially great, for it was 

 at his suggestion that the work was undertaken, and under his judi- 

 cious guidance and supervision it has grown to its present form. 



As is well known the natural habitat of the Isopods is in salt and 

 fresh water, along the shore or in the interior of the country, usually 

 in moist situations. They do, however, often occur in places most 

 remote and unlocked for. Some have been found in warm springs 

 and in subterranean streams, in caves and grottoes, pumps and wells, 

 and far from human habitation. The littoral forms are seen around 

 wharf piles, under rocks and stones, on ledges and cliffs, where the}' 

 are not easily captured, for they seek refuge, with surprising activity, 

 among the cracks and crevices. 



The curious habitations sought for shelter and protection are the 

 tubes of worms, the burrows of the crayfish, the nests of mollusks, 

 and the nests of ants. Some live in siliceous sponges, others in coral- 

 lines or among ascidians. 



The parasitic Isopods attack other Crustacea as well as fishes. The 

 parasites of fish are found on the skin, fins, gills, and in the mouths of 

 their hosts, and have even been known to bore holes in the body back 

 of the fins. One of these, Olencira prseyustator^ attacks the menhaden 



