-IfiO BULLETIN OF THE BUREAU OF FISHERIES. 



Valuable aid was received both in the way of specimens and literature from the 

 United States National Museum and the Bureau of Fisheries, and is gratefully 

 acknowledged. The photographs of the species illustrated in the plates were taken 

 in the zoological laboratory of the University of Michigan. 



The Amphipoda are found in practically all parts of the ocean. Many species 

 are confined to near the shore, where they live among rocks and seaweeds. Others 

 are strictly pelagic in habit, such as most of the Hyperiidea, which occur, often in 

 very great numbers, at or near the surface of the open ocean. The Gammaridea 

 also occur in great abundance, especially in the Arctic regions, where they assume, 

 as a rule, a larger size than in more southern waters. 



Little that is definite is known concerning the role played by the Amphipoda in 

 the bionomic relations of marine life, but there can be small doubt that it is an 

 important one. In addition to living upon the seaweeds and the bodies of dead 

 animals, amphipods actively prey upon smaller forms of life. In turn, they fall 

 victims to the rapacity of higher organisms. They are preyed upon by many kinds 

 of fishes, of whose food they constitute a not inconsiderable proportion. The variety 

 of their habitats and the great abundance they sometimes attain render them impor- 

 tant elements in the food supply of many higher marine animals. 



GENERAL CHARACTERS OF THE AMPHIPODA. 



Malacostraca, in which the body is divided into a head, a thorax of seven free 

 segments, and an abdomen, which consists typically of six segments and a telson; 

 no carapace; eyes sessile and usually compound; gills in the form of sacs attached 

 to the inner side of the. first joint of the thoracic legs; first three pairs of abdominal 

 appendages fitted for swimming; the last three pairs very different from the preceding 

 ones in structure, directed backward, and adapted for springing. 



With the exception of the terrestrial sand-fleas, belonging to the Orchestiid;v. 

 all of the Amphipoda are aquatic and the great majority of the species marine. 



EXTERNAL STRUCTURE OF AMPHIPODA. 



In order to facilitate the identification of species by those who may not be 

 familiar with this group of Crustacea, I have inserted the following account of those 

 structural features which are commonly used in classification: 



Divisions qftJu body. — The body of an amphipod crustacean is divisible into 

 three principal parts — head, thorax, and abdomen. The segments composing the 

 head are indistinguishably fused, and there is some difference of opinion regarding 

 the number of segments of which the head is constituted. It is certainly as many as 

 six; according to Delia Valle, and to some others, it is seven; and YVestwood puts 

 the number as high as nine. But there is not, I believe, sufficient evidence, either 

 anatomical or embryological, to justify us in recognizing more than seven cephalic 

 segments, if, indeed, that many. The term head, as Doctor Stebbing has remarked, 

 is one of rather loose application. What is termed the head in the Amphipoda 

 corresponds to the head plus the first thoracic segment in the Decapoda. In most of 

 the Amphipoda the head is very sharply marked off from the thorax. In one group, 

 however, the Caprellidea, the first thoracic segment is more or less completely fused 



