No. 26.] ARTHROSTRACA OF CONNECTICUT. 15 



PART I. AMPHIPODA. 



KEY TO THE SUBORDERS OF AMPHIPODA. 



The^Amphipoda are divided into three suborders which may 



be readily distinguished from each other. 



A. Eyes large, covering nearly the whole side of the head; uro- 

 pods usually with laminate rami which form a tail fan with 

 the telson HYPERIIDEA p. 44 



AA. Eyes usually present but never enormously developed. 



B. Abdomen well developed GAMMARIDEA p. 47 



BB. Abdomen rudimentary; body extremely slender 



CAPRELLIDEA p. 174 



ANATOMY 

 GENERAL FORM OF BODY AND TELSON , 



In all the Amphipoda except the Caprellidea, the general 

 aspect of the body is quite uniform, being slender and slightly 

 tapering at each end and usually compressed laterally. In only a 

 few genera, such as Corophium, Siphonoecetes, Unciola, and 

 Chelura, is the body cylindrical or depressed. The Lysianas- 

 sidae and Ampeliscidse are the most compressed forms. The 

 Caprellidea are slender and threadlike. 



The body is divided into three regions : the head or cephalon, 

 the thorax, and the abdomen. The head never exhibits its 

 segmental structure and the number of segments represented in 

 this region is still in question; it is probably seven. The thorax 

 is made up of seven segments, and the abdomen of six, exclusive 

 of the telson, which is probably not of the nature of a regular 

 segment of the body. The abdomen always is made up of three 

 freely articulating segments anteriorly, followed by three which 

 are never very movable and may suffer more or less fusion ; thus 

 the fourth and fifth segments are fused in Dexamine and 

 Ampelisca and the fourth, fifth, and sixth are consolidated in 

 Chelura terebrans. This fusion, however, is of no great system- 

 atic value, since in different species of the genus Corophium, 

 for example, the last three segments may be either fused or all 

 distinct. In the Caprellidea the abdomen is very rudimentary and 

 is generally devoid of any indication of segmentation. 



