REPORT ON THE CRUSTACEA MACRURA. 469 



and most probably is, the first appearance of the larger tooth that is so conspicuous a 

 feature in the adult male. As yet, there is no evidence, even in a rudimentary condi- 

 tion, of the petasma that forms so important a character in the adult male. 



Group ABEERANTIA. 



The species that are placed in this group differ from those belonging to the Normalia 

 of this division of the Macrura, chiefly in the varying and eccentric condition of the 

 pereiopoda. 



In the Penaeidea, these appendages in the highest pronounced types are well developed 

 and constant in form, consisting anteriorly of three chelate pairs, and posteriorly of two 

 simple pairs, but these all diminish in value and importance, as they descend in the scale 

 of the various families. 



The two posterior pairs of pereiopoda first appear to suffer degradation and become 

 long, slender, and feeble appendages, useless as organs of locomotion either for walking or 

 swimming. This is apparent in the genera Benthesicymus, Gennadas and Benbhecsetus, In 

 the two former they are long, slender and styliform, whereas in Benthecsetus they are 

 long, slender, filiform and multiarticulate. In the Sergestidae they become still further 

 depreciated, losing much of their power in Sergestes, and altogether disappearing in the 

 genus Acetes. 



In this latter family not only do the two posterior pairs disappear, but the others also 

 suffer considerable degradation, the number of the joints of the first pair of pereiopoda 

 being reduced to six, and it loses its chelate state, and the two following pairs are 

 reduced to a feeble and exhausted condition, and although the chelae are retained, 

 they are microscopical in proportions. 



But with this degradation of the pereiopoda we find an increased power and im- 

 portance given to the gnathopoda. The first pair, especially, becomes a strong and 

 powerful appendage, furnished with a genuflexed carpal joint, which cannot be 

 extended straight, and indicates its usefulness as a powerful organ of prehension. 

 The second pair also is robust, more especially at the basal joints, and is also a long and 

 powerful organ. 



In the Luciferinae, as in the genus Acetes, the two posterior pairs of pereiopoda have 

 disappeared altogether. The first pair of pereiopoda also has still further diminished 

 in value than in the Sergestinse, and the second pair has lost its chelate character, a 

 feature that is only represented by the minute chela of the third pair. 



In this division the branchiae vary from a condition in which they are the most 

 numerous and finely developed in the order, as in Benthesicymus, to their entire 

 disappearance, as in Lucifer. 



