PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxvii. 



have been of great service in preparing the general account of the 

 group and freely used are those of T. R. R. Stebbing, G. O. Sars, 

 F. Beddard, O. Harger, C. E. A. Gerstsecker, J. P. McMurrich, J. C. 

 Schicedte and F. Meinert, James D. Dana, A. Giard and J. Bonnier, 

 and H. G. Hansen. A large number of other papers have also been 

 consulted. 



Finally, I wish to express my gratitude to Dr. Theodore Gill, the 

 distinguished naturalist, for his kindness in looking over the work 

 and for his many suggestions in connection with it. 



INTRODUCTION. 

 1. CLASSIFICATION AND SYSTEMATIC POSITION. 



The Isopoda are a group of Crustacea belonging to the large subclass 

 Malacostraca, which, in contradistinction to the Entomostraca, includes 

 all those forms having a gastric mill in the stomach; green glands, 

 functioning as excretory organs, situated in the basal joint of the 

 antennules; and not having a free nauplius larva, the nauplius stage 

 being passed in the egg. 



The order Arthrostraca includes both the Isopoda and the Amphipoda, 

 which have in common the following characters: The first thoracic seg- 

 ment is permanentl}^ fused with the head, and bears maxillipeds; the 

 seven remaining segments are usually free and bear legs, although the 

 first free segment and sometimes even the second free segment may 

 be united with the head to form a carapace; the eyes are usually sessile. 



The Isopoda difi'er from the Amphipoda (1) in the general form of 

 the body which is compressed dorso-ventrally, while in the Amphipoda, 

 it is flattened or compressed laterally; (2) in the fact that respiration 

 is carried on by the abdominal appendages or pleopoda, which are 

 modified gills, while in the Amphipoda the gills are borne on the 

 thoracic appendages; (3) in the difference in structure correlated with 

 a difference in function in the pleopoda. The abdominal appendages 

 in the Isopoda are usually broad plates or lamellae, all five pairs more 

 or less similar in shape and size. In the Amphipoda, the first three 

 pairs are similar in shape and size, are long narrow appendages suitable 

 for swimming, while the last three pairs are adapted for jumping. 



Seven superfamilies" or tribes are usually recognized in the sub- 

 order Isopoda. The classification given by G. O. Sars is the one now 

 generally adopted. He divides the Isopods into tribes according to 

 the following characters:^ 



aThe term tribe was originally used by Latreille for subdivision of family, and 

 such was for a time the general usage. The history of the group has been indicated 

 by Dr. Gill in his address on Some Questions of Nomenclature (Science, n. s. IV, 

 p. 598, etc.). 



6 The table has been modified to include the tribe Phreatoicoidea. Other slight 

 changes have been made also. See Sars, Crustacea of Norway, II, 1899, p. 3. 



