GENEALOGY OF CRUSTACEA. 



417 



46. — Advanced 

 larva of Lepidurua en- 

 larged. After Brauer. 



tlieir xJalPBontoiogical history, the latter being an important check upon 

 any errors arising in the former. 



^ it has seemed to us the more natural view that the systematic posi- 

 tion and relations of the 

 Phyllopods, as compared 

 with the Cladocera, is that 

 tbe PhyHopods are simply 

 a highly developed and ex- 

 tremely specialized branch 

 of aCIadoceroussteni; that 

 the Cladocer.T. are a step 

 higher than the Ostracoda, 

 which connect the Bran- 

 cMopoda with the Copepoda. 

 There is a tolerably com- 

 plete ascending series of 

 forms, beginning with the 



r^ 1 11- „+,• . Fig. 47. — ^auvilma of Branchi- 



Copepoda and culminating ^^sstaf^nazis. After ciaus. 

 in the Phyllopods. Here 

 we should stop, and in endeavoring to account for the origin of the De- 

 capoda, we do not see what facts there are to sustain the view that the 

 highly specialized Decapoda, much less the Tetradecapoda, originated 

 from the Phyllopods or forms like them. The more natural view is that 

 the Malacostraca originated by a direct line of ancestral forms, resem- 

 bling the zoea, protozoea, «&c., beginning with a ISTauplius condition ; 

 the development of Peno3us and Leucifer giving us data for such a hy- 

 pothesis. 



Hence the Phyllopods and Decapods, for example, for a time proba- 

 bly followed the same developmental path or rather parallel paths. The 

 Phyllopods, culminating in the highly specialized peculiar type of Apo- 

 didw and especially the BrancliipodldcG^ were the flowering out or con- 

 sumation of, so to speaiv, the Branchipodous branch of the Neocaridaii 

 crustacean tree. On the other hand the Decapods, beginning with the 

 JSTauplius form, i^erhaps more rapidly and by an accelerated course of 

 development comparatively late in l)al^^eo^tological history, assumed the 

 primitive Decapodous characteristics perhaps before the Phy]lo])odous 

 type had been perfected, but in the Tertiary Period culminated in a great 

 profusion and luxuriance of forms, remarkable for the number of species 

 and variety of shapes of macrourous and especially brachyurous types. 



The palteontological history of the TSTeocarida, as we have endeavored 

 to show by the diagram ou p. 361, shows that the shrimps existed dur- 

 ing the Devonian, 1 that the crabs wi-re already in existence during 

 the Carboniferous Period, before the Apodidw and BrcmcMpodidm had, 

 judging by their fossil remains, aiipeared ; while the Limnadiadce, gen- 

 uine Phyllopods, ap])cared before any Decapods in the Devonian, the 

 Ostracodes being abundant in the Lower Silurian strata. It seems to us 

 therefore most probable from a geological standpoint that the Decapods 

 could not have originated from the Phyllopods, as the two types were 

 developed during the Palseozoic era. 



That the Phyllocarida were developed independently either of the 

 Phyllopods or of the Decapods seems probable from the fact that the 

 Phyllocaridan type became established as early as the Lower Silurian. 

 We shall see that the Phyllocarida are not related to the zoea of Dec- 

 apods, and that the Decapods probably did not originate from them. 



^ Palvsoimlcemon neivlerryn lias been described by Wliitfield from the Devonian of 

 Ohio. 



27 H 



