MARIN© GAMMARIDEAN AMPHIPODA 5 



precisely at right angles to each other but also seems to have reference 

 to a quadrate shape with softly rounded corners. 



The terms "vestigial" and ''rudimentary" have their uses, for a 

 mathematical definition in each case might be more involved, con- 

 fusing, and time consuming than is the process of learning these 

 conditions through trial and error. 



A decision as to whether a gnathopod is simple or subchelate 

 is occasionally difficult to make and definitions to classify all border- 

 line cases are almost impossible to compose. There are no terms to 

 describe all of the stages between fully subchelate and fully simple 

 and gammaridean students may eventually encounter difficulties 

 arising from dogmatic use of such all-or-none terms. For instance, 

 two subfamilies of Lysianassidae have been described which are 

 partially based on a subchelate or simple gnathopod 1, but there are 

 several borderline cases and several genera in one subfamily which 

 seem to have stronger affinities with genera in the other subfamily 

 than with members of their own group. The contrast between the 

 concepts of cleft telson and entire telson is far too strong to describe 

 all intermediate stages and the two terms tend to condition the 

 human observer to a reliance on the alternatives as highly significant, 

 whereas numerous exceptions to this significance are apparent [e.g., 

 Eusiridae and Calliopiidae]. 



Names for the gammaridean abdomen and its parts are not stand- 

 ardized. All six segments of the abdomen are frequently called the 

 "pleon", but "pleonites" 4-6 are often called "urosome" and num- 

 bered as "urosomites" 1-3. Pleonites 1-3 are rarely called the 

 "metasome." 



Shape and proportion in seemingly infinite array are thus more 

 important to the taxonomist than are qualitative and numerical 

 expressions, but one day we may be able to convert shape and pro- 

 portion to precise formulations. 



Morphology of a Gammaridean Amphipod 



Amphipods, like tanaids and isopods, lack a carapace covering 

 the thorax, so that seven definitive thoracic segments (pereonites) 

 are visible (as in other Peracarida the first thoracic segment with its 

 appendage [maxilliped] has become fused to the head). Gammaridean 

 and hyperiidean Amphipoda may be recognized by their possession 

 of three pairs of pleopods (swimmerets) and two or three pairs of 

 uropods on the pleon (abdomen). The consistent presence of at 

 least six pairs of thoracic appendages, five-plus pairs of gills and 

 four pairs of brood lamellae in females are definitive characters of 

 Gammaridea and Hyperiidea. 



