PHYLUM ARTHROPODA 



321 



mesosoma was of six segments and bore ventrally an operculum on the 

 first, and five pairs of flattened plates, believed to have been gills, on 

 the others. The rnetasoma of seven segments lacked appendages, and 

 ended in a telson spine. 



Although eurypterids are believed to have been primarily a fresh- 

 water group, the evidence for this is not conclusive. The best deposits 

 of fresh-water organisms are usually found where they have been washed 

 into the sea at the mouths of rivers. Although many of the fossils in such 

 deposits are obviously of fresh-water origin, others are just as clearly 

 marine. 



Arachnida. In the class Arachnida the most primitive order, Scor- 

 pionida, shows many similarities with the preceding classes. The scorpion 

 (Fig. 16.29) has a prosoma with six pairs of appendages, the first of which 

 are the chelicerae. The second pair are large and chelate, forming pincers 

 comparable with those of the crayfish. The remaining four pairs are 

 walking legs. 



The scorpion mesosoma has six segments, of which the first has a 

 small bilobate appendage now part of the reproductive apparatus and 

 thought to be a vestigial operculum. The second segment bears a pair 

 of combs, modified tactile limbs. The third to sixth segments each bears 

 a pair of ventral slits that open into air chambers containing book lungs 

 formed of many delicate lamellae. Embryological evidence suggests that 

 these lungs are borne on limb vestiges that have sunk into the body, 

 protecting the lamellar respiratory organs from desiccation. 



The scorpion metasoma is made of one tapered segment and five 



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ARACHNIDA 



Figure 16.29. Representative classes of the subphylum Arachnomorpha. Two of the 

 arachnid orders are shown. A third class (Xiphosura) is shown on Figure 16.2. (Combined 

 from various sources.) 



