MARLNTE GAMMARIDEAN AMPHIPODA 105 



Order Amphipoda 



Diagnosis. — Peracarid Malacostraca lacking carapace, thus having 

 all but one or two of the thoracic segments freely visible; one thoracic 

 segment carrying maxillipeds fused to head, occasionally next thoracic 

 segment carrying gnathopods also fused to head (Caprellidea), 

 followed by seven (occasionally six) visibly articulated thoracic seg- 

 ments, each bearing paired appendages, followed by six abdominal 

 segments or their macroscopic remnants (except Caprellidea), first 

 three (pleon) usually bearing paired biramous pleopods, remaining 

 three (urosome) bearing paired biramous uropods ; telson freely articu- 

 late in primitive and majority of members; head with two pairs of 

 antennae, first occasionally biramous; maxillipeds lacking exopodites; 

 heart mainly thoracic ; respiration thoracic with gills attached to coxae 

 (or their remnants) of segments 2-7 (variable) ; eyes sessile or rarely 

 borne on unstalked cephalic scale; eggs carried in female brood pouch 

 on ventral thorax formed of 2 to 4 (or 5) pairs of lamellae attached 

 to coxae 2-6. 



There are no radically degenerate and fully endoparasitic Gam- 

 maridae, although a number are inquilines, ectoparasites, and com- 

 mensals which have sucking mouthparts and prehensile mechanisms 

 on their appendages. 



Gammaridea, especially the Gammaridae, lie closest to the logical, 

 primitive stem of the Amphipoda and almost aU evolutionary lines 

 from Gammaridae-like ancestors are based on simplification of primi- 

 tive parts. The Caprellidea have reduced but a vestige of the abdomen 

 (except Cercops and Caprogammarus) and the Hyperiidea have lost 

 the maxillipedal palps. Reduction of coxae is common in both of those 

 suborders. Ingolfiellidea have lost all pleopods but some have devel- 

 oped a movable scale on which the eyes are borne when present. 



One family of Gammaridea, the Ochlesidae, have lost the maxilli- 

 pedal palps but their resemblance to Gammaridea in coxae and body 

 shape and their presumed benthic habits have caused their assignment 

 to the Gammaridea. 



Key to the Suborders of Amphipoda 



1. Abdomen normally vestigial, usually lacking large pleopods or uropods, except 

 for microscopic vestiges not exceeding two pairs, gills two or 3 pairs and 

 brood lamellae two pairs each, head and segment bearing gnathopod 1 

 usually immovably coalesced. 



CAPRELLIDEA (skeletonshrimps, whalelice, 250 species) 



Abdomen well developed, with six segments or their gross vestiges apparent, 



bearing some gross appendages, gills and brood lamellae exceeding three 



pairs each, head and thorax free 2 



