Diplopoda — Arachnida ^i^'j 



Millipedes closely like those now living have been 

 found in Oligocene rocks, while remains of their 

 progenitors, showing the gradual progress of the 

 compound segments, can be traced through the Prim- 

 ary formations from the Devonian to the Permian. 

 The recent discovery (2I0) of an embryonic segment, 

 distinct, though without appendages, behind the 

 maxillary segment, goes far to bring the head of a 

 millipede into correspondence with that of a centi- 

 pede or an insect ; and we can probably regard the 

 class as springing from ancestors something like Scolo- 

 pendrella. That the affinity of Centipedes and Insects 

 to each other is greater than that of either to the 

 Millipedes is shown by the position of the genital 

 openings being far forward in the body among the 

 latter animals (as in Scolopendrella) and near the 

 hinder-end in the two former classes (209). The 

 fact that some millipedes are hatched from the egg 

 as six-legged larvse has often been urged in support 

 of their close relationship with Insects. But these 

 limbs cannot correspond with the six legs of an insect 

 since they are not borne on successive segments, and 

 the segments whereon they do occur are not always 

 constant in the various species of Millipedes. 



Arachnida. — The Scorpions, Spiders, Mites, and 

 other orders of Arthropods which make up the class 

 Arachnida are sharply marked off from ^lillipedes, 

 Centipedes, and Insects, by the absence of a distinct 

 head and of feelers. The head and fore-body seg- 

 ments are fused together, at least in part, to form a 

 cephalothorax which carries six pairs of appendages, 

 whereof the first two are usually seizing-jaws, and 

 the last four walking-legs. It is likely that these 

 six pairs of limbs correspond with the jaws and legs 

 of insects, since vestiges of at least one pair of feelers 

 and two or three head-segments without appendages 



