[ 320, ] 



IX. 



Oy THE EELATIOTs'SHIPS BETWEEX THE CLASSES OF 



THE AETHEOPODA. 



Bx GEOEGE H. CAEPEXTEE, B.Sc. LoncL, il.E.I.A., 



of tlie Science and Art Museum, Dublin. 



[Plate VI.] 

 EeadMATll, 1903. 



Introduction. 

 Fe"w zoological problems have given rise to Avider differences of 

 opinion than tbat of tbe relationships that may exist between the 

 various classes of animals included under the term " Arthropoda." Eor 

 many years, the existence of some rather close affinity between Insects 

 Centipedes, lEillipedes, Arachnids, and Crustaceans was undisputed. 

 Linne, in 1758, included all these groups in his " Class Insecta"^; and 

 the name " Ai'thropoda," bestowed upon the assemblage by Yon Siebold 

 in 1848, was intended to mai'k themofi as a gi-and primary division of 

 the Animal Kingdom. "When the evolutionaiy doctrine spread, and 

 naturalists began to go ancestor-hunting, there was no hesitation in 

 deriving all the Linnean "Insecta" from a common stock. The 

 development of so many and diverse Crustacea from a Xauplius larva 

 was believed by Miiller ('69) to indicate the descent of the whole 

 Crustacean class from xs^auplius-like ancestors ; and through some 

 primitive Phyllopod, the Arachnida were traced back to the same 

 parent-stem. The six-legged larva of certain JSJillipedes led to the 

 conclusion that both Insects and " Myriapods " had originated from a 

 Thysanuriform stock, which had been derived, according to Haeckel 

 ('76) and others, from a primitive zoaea-like Crustacean. 



During recent years these phylogenetic speculations have been 

 discredited by many zoologists. If too much weight was formerlv 

 allowed to larval stages in the discussion of ancestral forms, the 

 tendency at present is to regard such stages as of hardly any im- 

 portance at all. Then comparisons have constantly been made 

 between Arthropodean and Annelidan organs — between appendages 

 and parapodia, coxal glands and nephridia, tracheal tubes and dermal 

 glands, so that many zoologists think it more instructive to compare 

 various Arthropods with Annelids than with each other. And the 



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