TYPE AEACHNIDA. 



457 



accordingly, that relationships to other forms must be looked for, and a 

 comparison of them with Limulus reveals similarities of structures so 

 numerous and so detailed that the conclusion is unavoidable that both are 

 to be traced back to a common ancestor. Thus the cephalothoracic ap- 

 pendages in both are identical in number, and, so far as the first two pairs 

 are concerned, in general structure also, while the genital opercula of the 

 .Scorpions are comparable in their relation to the genital orifices to the 

 opercula of Limulus, and the pectines to the first pair of abdominal ap- 

 pendages. The remaining abdominal appendages of Limulus, which are 

 branchiate, seem at first sight to be unrepresented, but the embryo- 

 logical investigation of the Scorpions appears to indicate that they are 

 represented by the lung-books, which bear no little resemblance to the 

 branchial lamellae of Limulus, and the 

 conversion of one set of organs into the 

 other may be supposed to have been brought 

 about by the formation behind each pair 

 of abdominal appendages of an invagina- 

 tion, which, deepening, has carried in with 

 it the branchial lamellae, the original an- 

 terior surface of the appendage forming 

 the ventral wall of the body beneath the 

 lung-sac, while the lamellae project into the 

 sac for its ventral surface (Fig. 210). In 

 the general form of the body Limulus 

 corresponds fairly well with the Scorpions, 

 the cephalothoracic regions being strictly FJG _ m _ DlAOKA ^; F 



OP LUNG-BOOKS (after KINGS- 

 LEY). 



/= indifferent stage. 



L = Limulus stage. 



A Arachuiclau stage. 



comparable, as is also the terminal spine 



with the sting ; the abdomen, however, in 



the branchiate form has a smaller number 



of segments which are all fused, a difference 



readily explained by the probable derivation 



of both forms from Eurypterus-like ancestors in which the abdomen 



possessed a relatively large number of distinct segments, and even showed 



indications of a differentiation into a pra?abdomen and a postabdomen 



(see Fig. 198). 



In the internal structure quite as striking similarities are to be found 

 in the presence of an endosternite in both groups and of coxal glands in 

 connection with the fifth pair of appendages, in the tendency towards the 

 concentration of the postoral ganglia, and in the .invaginate origin of the 

 median eyes, to mention but a few points. 



The Arachnida are accordingly to be traced back to Limulus or Euryp- 

 terus-\\V.Q ancestors, and through these finally to the Entomostraca, perhaps, 

 a Crustacean ancestry being clearly indicated. As to the relationships of 

 the various orders little that is definite can be said, differentiations having 

 taken place along different lines in the various orders, so that while the 



