4()4 



THE AMERICAN NATURALIST 



[Vol. LIV 



can no longer be held that phyllopodan limbs are primi- 

 tive, and, stripped of their trilobite-like disguise, these 

 wormlike crustaceans may no longer be considered most 

 primitive. The possession of biramous limbs by the 

 branchiopods of the Middle Cambrian, added to their 

 other undoubted likenesses, indicates the possibility that 

 they were derived from the trilobites, although some of 

 them had then already attained the specialized carapace, 

 pedunculate eyes, and limbless hind-body. This possi- 

 bility is converted into strong probability when one con- 

 siders the structure of the beautiful Marrella splendens 

 Walcott. The head of this "lace-crab" of the Middle 

 Cambrian is obviously highly specialized, but the struc- 

 ture as a whole proves it to occupy an intermediate posi- 

 tion between the trilobites and more specialized crusta- 

 ceans, including the branchiopods. It resembles the 

 higher crustaceans in having the antennae uniramous, in 

 lacking exopodites on the cephalic appendages, gnatho- 

 bases on those of the thorax, and in the absence of pleural 

 lobes from the test of the trunk. This animal retains 

 enough characteristics of the trilobites to show that it 

 was derived from them, and has attained enough charac- 

 teristics of the higher Crustacea to show that it belongs 

 with them. A better connecting link can hardly be ex- 

 pected. 



The Copepoda prove, on analysis, to be much more 

 closely allied to the trilobites than had been supposed. 

 All students have remarked upon the many primitive 

 features of the non-parasitic members of this group, but 

 have generally explained them by the sweeping assertion 

 that they must be degenerate. Why, if they are degen- 

 rate, do the Copepoda show fewer modifications during 

 development than any other Crustacea except the trilo- 

 bites? They, instead of Apus, represent the "grown up 

 nauplius. ' ' 



These animals resemble the trilobites in lacking a cara- 

 pace, in possessing pleural lobes, which, however, are in- 

 curved instead of being flattened. The greatest resem- 



