STRUCTUEE AND CLASSlFICATlOxN OF THE ARTHROPODA. 563 



same tissue elements. In Peripatus the stigmatic pits at 

 wliicli the trachea} communicate with the atmospliere are 

 scattered and not definite in their position. In other cases 

 the stigmata are definitely paired andphtced in a few segments 

 or in several. It seems that we have to suppose that the 

 vasifactive tissue of Arthropoda can readily take the form of 

 air-holding instead of blood-holding tubes, and that this 

 somewhat startling change in its character has taken place 

 independently in several instances — viz. in the Onychophora, 

 in more than one group of Arachuida, in Diplopoda, and, 

 again, in the Hexapoda and Chilopoda. 



The Malpighian Tubes. — This name is applied to the 

 numerous fine ctecal tubes of noticeable length developed from 

 the proctodfeal invert of ectodermal origin in Hexapods. 

 These tubes are shown to exCrele nitrogenous waste products 

 similar to uric acid. Tubes of renal excretory function in a 

 like position occur in most terrestrial Arthropoda — viz. in 

 Chilopoda, Diplopoda, and Arachnida. They are also found 

 in some of the semi-terrestrial and purely aquatic Amphipod 

 Crustaceans. But the conclusion that all sucli tubes are 

 identical in essential character seems to be without founda- 

 tion. The Malpighian tubes of Hexapods are outgrowths of 

 the proctodeum, but those of scorpion and the Amphipod 

 Crustacea are part of the meteuteron or endodermal gut, 

 though originating Jiear its junction with the proctodteum. 

 Hence the presence or absence of such tubes cannot be used 

 as an argument as to affinity Avithout some discrimination. 

 The scorpion's so-called Malpighian tubes are not the same 

 organs as those so named in the other Tracheata. Such renal 

 csecal tubes seem to be readily evolved from either meteuteron 

 or proctodaium when the conditions of the oiitwash of nitro- 

 genous waste products are changed by the transference from 

 aquatic to terrestrial life. The absence of such renal casca in 

 Limulus and their presence in the terrestrial Ai-achnida is 

 precisely on a parallel with their absence in aquatic Crustacea 

 and their presence in the feebly branchiate Amphipoda. 



We shall now pass the groups of tiie Arthropoda in review, 



