1. BRIEF MORPHOLOGICAL OUTLINE 



As in other amphipods, the hyperiidean body is divisible into three sec- 

 tions: head, thorax, and abdomen. The cephalic section, or the head 

 (cephalon), consists of the acron and five body articles completely fused 

 with each other. It bears six paired appendages — antennae I (or anten- 

 nules), antennae II, mandibles, maxillae I, maxillae II, and maxillipeds, 

 as well as an unpaired labrum and labium. 



The thoracic section (pereon or mesosoma) consists of seven 

 somites. Each thoracic article bears a pair of walking legs (pereopods). 

 The first two pairs of these walking legs in gammarideans are provided 

 with chelae, and are called gnathopods. In many hyperiideans, these 

 legs are simple and do not differ essentially from the successive ones; 

 hence it is customary to name all walking legs as pereopods I-VII. The 

 coxal article of the sympodite articulates with the lateral margin of the 

 corresponding somite and is modified into a coxal plate dorsally covering 

 the proximal part of the leg. 



The abdominal section consists of six somites. The first 

 three (pleon or metasoma) bear multisegmented biramous natatory 

 legs — the pleopods. The last three somites (urosoma) have biramous 

 appendages — the uropods — with unsegmented rami. On the distal end 



15 of the last abdominal somite, there is a freely movable telson, which is 

 always entire in hyperiideans. Sometimes the telson is fused with the 

 urosome (Figs. 1 and 2 ). 



The somites of the pereon are usually free but in some genera the first 

 two (Acanthoscina, Spinoscina, Hyperietta, Hyperioides, Phronimopsis, 

 Dairella, Phronimella, Phrosina, Anchylomera) or three (Hyperionyx), 

 four (Lestrigonus) or even five somites (Themistella) may be fused with 

 each other. Similarly, somites II and HI of the urosoma may also be 

 fused. 



In the species of some deepwater families (Lanceolidae, Microphas- 

 midae, Cystisomatidae), the integument is very thin and transparent; con- 

 trarily, in fast surface swimmers such as the Pronoidae, Platyscelidae, and 

 Parascelidae, it is hard and pigmented. 



The body shape is highly variable. Many surface forms have a 

 short, compact body with well-developed musculature. Some of them 

 (Platyscelidae, Parascelidae) can roll themselves into a firm, quick- 



16 sinking ball, sometimes armed with pointed spines formed by the 

 processes of coxal plate V (Platyscelus armatus). In many deepwater 

 Physosomata, particularly in females, the pereon is modified into an 

 inflated sphere with a small pleon and short, contracted appendages 

 (Archaeoscina, Mimonectes); in others the pereon is slightly dilated 

 laterally and thickened dorsoventrally, which is characteristic primarily 



