3 



68 The Pedigree of Insects 



have been detected in the embryo of a spider (2Il). 

 In the Jower Arachnids, the Scorpions (fig. 182) for 

 example, there are twelve distinct segments in the 

 hind-body ; and limbs on certain of these segments 

 are preserved in many living arachnids, the pectines 

 of Scorpions, the gill-plates of Limulus, and the spin- 

 nerets of Spiders, being well-knovi^n examples. There 

 is much reason therefore for believing that arachnids, 

 like insects, have been derived from an ancestor with 

 nineteen limb-bearing segments. Many arachnids, the 

 higher Spiders and the Mites, for example, breathe, as 



Fig. 182. — Typical Arachnid — a Scorpion (Pa/avmcpus Sivant- 

 merdamii). From Pocock, Natural Science, vol. 9. 



insects do, by means of tracheal air-tubes, and on this 

 ground a rather close relationship between the two 

 classes has been advocated by some students (214, 2l6). 

 But the more generalised arachnids either live in the 

 water, breathing by gills on someof the abdominal limbs 

 (Limulus and the extinct Eurypterids) ; or are land- 

 animals (Scorpions, Pedipalps and the lower Spiders), 

 breathing air by internal "lung-books" which agree in 

 position and essential structure with the gills of their 

 marine relations (208, 215)- It seems clear therefore 



