10 U.S. MATIONAL MUSEUM BULLETIN 271 



five pairs of pleopods on segments 1-5, but in noncaprellidean amphi- 

 pods the appendages of segments 4 and 5 resemble the terminal 

 uropod and thus are called uropods 1 and 2. Uropods in many mala- 

 costracans are still used for swimming although in different fashion 

 from pleopods, but uropods 1-2 in Amphipoda appear to be used 

 primarily for strengthening the caudal portion of the body to permit 

 jumping or flipping by rapid flexion of the urosome. In many Gam- 

 maridea, the third uropods still bear "swimming" setae, and may 

 be used for paddhng or as rudders. Males especially have natatory 

 third uropods. But the vast majority of Gammaridea probably do 

 not use the third uropods for active swimming and they are often 

 reduced or occasionally absent in sedentary species. Caprelhdea have 

 lost all but a vestige of the abdomen and its appendages. 



The telson is a flap attached to the sixth pleonite above the anus. 

 It is of primary taxonomic value, depending on whether it is cleft 

 into two lobes, fused into a single flap, elongate, fleshy, or ornate. 

 Possibly, it is a vestige of paired appendages. 



The head bears two pairs of antennae. The first three articles 

 of the first pair are known as the peduncle, the remaining smaller 

 articles the flagellum. In many species an accessory flageUum demon- 

 strates the biramous derivation of the appendage; when present, 

 it sprouts from the end of the third peduncular article and may be 

 elongate or reduced to three, two, or one articles. Although appearing 

 to be of minor importance, the condition of the accessory flageUum is 

 crucial to amphipod systematics and is useful especially at famihal 

 and generic levels. The second antennae bear five peduncular articles, 

 followed by a single flagellum. The flagella of both antennal pairs 

 may bear, especially in males, sensory appendages, such as aesthetascs 

 and calceoH. Male antennae often are longer than those of females. 



Frequently famihes and genera have been defined as lacking 

 accessory flagella and then have been shown to have some members 

 bearing extremely small 1-articulate pieces. Allowance should be 

 made by the observer so as to admit to genera and families those 

 species with microscopic remnants of accessory flageUa that have 

 heretofore been overlooked. 



The mouthparts are composed of the following structures; they 

 are highly variable intergenerically and their morphology is important 

 for classification: 



Upper lip: A single lobe or flap anterior to the mouth. In about 

 10 percent of known species the anterior cephahc surface above the 

 upper lip is produced into a point, keel, or lobe known as the epistome. 

 Its function is unknown. In a few famihes, especially Lysianassidae, 

 the upper Up has a keel projecting anteriorly and usually separated 

 from the epistomal region by a deep sHt or sinus. Occasionally both 



285-135 O - 69 



