THE 



AMERICAN JOURNAL OF SCIENCE 



[THIRD SERIES.] 



Art. XXXIII. — Observations on the Derivation and Homolo- 

 gies of some Articulates ; by James D. Dana. 



The term Articulates is used here in preference to Arthro- 

 pods because the latter group is believed to be not a natural 

 one, Crustaceans and Insects being less closely related to one 

 another, as indicated beyond, than Annelids and Insects. 



Derivation of Limuloids and Crustaceans. — As has been 

 suggested by Lankester, it is probable that all the Articulates 

 are successional to the Rotifers. There is reason for believing 

 further that the types of Annelids, Crustaceans, and probably 

 that of Limuloids, had their independent Rotifer origin. 



The Nauplius, or larval form of a Crustacean, shows, by 

 its having but 3 pairs of limbs (2 besides an antennary pair) 

 that the type is not successional to a many-jointed Annelid, 

 but rather to some Pedalion-like Rotifer. The discoveries of 

 Prof. C. E. Beecher announced in the preceding and earlier 

 numbers of this Journal leave no doubt that the Trilobites are 

 multiplicate Isopod Crustaceans, precursors of the normal 

 Isopods, as the true Phyllopods, also multiplicate species, were 

 precursors of the Decapods.* 



* In the Author's Report on the Crustacea of the Wilkes Exploring Expedi- 

 tion, the Rotifers are made the lowest subdivision of Crustacea (p. 1408); and 

 the Trilobites are placed, with a query, in the subdivision of Tetradecapods as 

 multiplicate forms under the type. In the text above, the expression true Phyllo- 

 pods is used, because most of the so-called Phyllopods of the Paleozoic exhibit, 

 in the specimens, no evidence that they are multiplicate, that is, have an exces- 

 sive or abnormal nuoiber of body-segments or appendages. 



Am. Jotjr. Sci.— Third Series, Vol. XLVII, No. 281.— May, 1894. 

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