A M P H I P D A. 



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male is very nearly as broad as the first thoracic segment. The six 

 posterior thoracic segments are truncate either side. 



Tribe III. AMPHIPODA. 



The thoracic position of the branchial appendages, and the styli- 

 form or non-branchial character of the three posterior pairs of abdo- 

 minal appendages, distinguish all the Amphipoda from both the 

 Isopoda and Anisopoda. 



Among the Amphipoda, two types of structure, as regards the organs 

 of the mouth, are recognised, and the distinctions of the two sections 

 thus indicated, extend to various parts of the body. In one type, the 

 outer maxillipeds are small and operculiform ; the eyes are large, the 

 facets covering the greater part of the large head, and giving the 

 animal a wild, staring look ; the extremity of the abdomen is broad and 

 depressed, and the natatory abdominal appendages are usually oval 

 lamellar. Such are the species of the Hyperia group — the Hyperidea. 



In the other type, the outer maxillipeds are elongated and palpi- 

 form, the eyes are small, the head of moderate size, the abdomen, 

 when not obsolete, narrow, and the natatory abdominal appendages 

 usually slender. 



Under this second type all the Amphipoda, excepting the Hyperise, 

 are included. They comprise, however, two distinct groups, based 

 upon as important characters as those which separate them from the 

 Hyperidea. These differences are most prominently exhibited in the 

 abdomen. In one section, the Caprellidea, embracing Caprella, Cya- 

 mus, and related genera, the abdomen is obsolescent. In a second, 

 the Gammaridea, the abdomen is fully developed, with three pairs of 

 natatory appendages, and as many of stylets. This second section 

 embraces the typical Amphipods, the Gammari, Talitri, and the like. 

 The abdomen in these species is usually a powerful organ of motion. 



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