770 



AETHROPODA 



PHYLUM VII 



segments (or somites). Some of these, however, may be suppressed during 

 the ontogeny, not all of them persisting to the adult stage, or else becoming 

 fused in various ways. 



The body segments of Arachnida are grouped together in higher aggregates 

 or categories, called " tagmata " by Lankester (or expressed more simply as 

 " regions "), of which three are usually distinguishable. These regions are : 

 (1) the prosoma, often termed also the " cephalothorax " ; (2) the mesosoma, 

 also called by some writers the "thorax" or " preabdomen " ; and (3) the 



A B 



nana 



stern urn 



-■3 



en I la I 

 aperi:ulum 



--- 3 



Vn 



'3 



mala 



aculeus. 



Fin. 1496. 



Duthus vccitarms Amoreux, a ty]iical OM-World Scorpion. A, Dorsal, and B, VRntral view. 



i/;i (after Kraepelin). 



metasoma (another name for the " postabdomen " or " tail " of earlier writers. 

 Fig. 1496). The first of these regions includes all of the segments in front 

 of the genital pore, usually six in number. The second, or mesosoma, begins 

 with the somite bearing the genital pore, and ends with the last somite which 

 bears free appendages, typically six segments in all. The third region, or 

 metasoma, consists usually of six segments, none of which bear appendages, 

 excepting that the terminal one often has attached to it a postanal "telson," 

 which may be considered as in the nature of an appendage. The latter takes 

 in Scorpions the form of the sting, in Xiphosures and Eurypterids that of the 

 spine. Among Merostomes, where the body is sometimes sharply divided 

 functionally into two regions only (" cei:)halothorax " and "abdomen" as 



