Clusfiijication of the Decapod Crustaceans. 461 



cliaracters, enumerated below, wliich separate the Natantia 

 from tlie Reptantia ; {A) in their early appearance in the 

 earth's history (probably in the Trias), tlioui^h it is true that 

 the remains of Reptantia aie found fully as early ; (5) possibly 

 in the structure of their gills, if, as Boas thinks, the phyllo- 

 branchia? of the Caridea and the trichobranchiae of the lower 

 Reptantia be both derived from the deudrobranchiEe of the 

 Pena?idea. In any case it is impossible to regard the phyllo- 

 branch condition as the original one, but whether dendro- 

 branchia3 or trichobranchiaj are to be regarded as the starting- 

 point of the gills of the Decapoda is much more doubtful. 

 It would be possible to support either theory by cases among 

 the Euphausiacea which might be regarded as substantiating it. 



Tlie evidence for the primitive nature of the Penaidea is 

 therefore strong, but it must not be supposed that the modern 

 Peiiaids were the stock from which the rest of tlie order arose. 

 Their loss of the appendix interna of the pleopods *, which is 

 found in Euphausiacea and in many Reptantia and Caridea, 

 is clear evidence that they do not stand in the direct line of 

 descent of the latter two groups. Moreover, the original 

 Decapoda must have borne the podobranch on the fourth leg 

 found in some of the lower Reptantia and the epipodite on 

 the last leg, of which Couti^re has found a vestige in many 

 Caridea. Both these structures have been lost by the 

 Penaeidea. Tlie most that can be said is that, of modern 

 Decapoda, the Penseidea more nearly approach the primitive 

 condition than any others. 



From the original Decaj)od stock, whose nearest descendants 

 ■we have Ibund in the modern Pen^eids, the Reptantia and 

 Caridea must have arisen separately, for it is impossible to 

 suppose that either of these specialized groups arose from the 

 other. They have no characters in common which they do 

 not also share with the Penreidea, and each, as we have seen, 

 has characters which it shares with the latter group and not 

 with the other. Tliere remains, then, the question, which 

 of the two was the first to leave the early Penajid stem, and 

 that this was the Reptantia is shown pretty clearly by the 

 following facts : — 



(1) The Caridea and Penaeidea have undoubtedly more in 

 common with one another than either of them has with the 

 Reptantia. This extends to characters which are at least not 

 obviously primitive, such as the " stylocerite " of the first 

 antenna. 



(2) The gill-series in the lower Reptantia are fuller 

 than in either Penaiids or Carids, so that it seems likely 



* Except on the second pair of the male. 



