780 



AETHROPODA 



PHYLUM VII 



■^--O' 



of segments in the three regions of the body, and the appendages of the prosonia 

 being identical in number and position. 



The general form of the body in Enrypterids (Fig. 1505) is somewhat like that 

 of a Scorpion, Imt is relatively broader ami shorter. The prosoma or cephalolhorax 



consists of six fused seg- 

 ments covered by a quadrate 

 carapace with its front 

 angles rounded. This beai-s 

 on its dorsal surface two 

 pairs of eyes — large kidney- 

 shaped lateral eyes, and 

 median ocelli. The com- 

 pound lateral eyes are 

 smooth in theEurypteridae, 

 facetted in the Pterygotidae. 

 As shown by Clarke and 

 Ruedemann, these facetted 

 eyes are identical in struc- 

 ture with those of Limulus. 

 On the ventral surface 

 of the prosoma (Fig. 1506) 

 are seen six pairs of appen- 

 dages, of which the first pair 

 (the chelicerae) are preoral 

 in position, and the remain- 

 ing five pairs are found at 

 the sides of the elongate 

 mouth, and are develojjed 

 as legs. These legs consist 

 typically of a basal joint 

 (coxa), the inner margin 

 of which (gnathobase) is 

 provided with teeth and 

 able to function in masti- 

 cation, much as in Limuhis 

 or Apu,s, whilst the distal 

 part of each appendage 

 served as an organ of loco- 

 motion. The ambulatory 

 part of the appendages is 

 usually six-jointed, and is 

 attached to a small, oval 

 "ej)icoxite" at its anterior angle exactly in the same fashion as in Limulus. The 

 fifth pair of legs is spineless and slender, probably serving as balancing organs. 

 The sixth pair is characterised in all members of the order by its greater size and 

 usually somewhat flattened form, as well as by its termination in an oval plate or 

 claw. This last pair is commonly termed the palette or paddle, and seems to have 

 had a swimming function, although it is probable that the animal used it also 

 for anchoring or burying itself in the mud. 



Behind the prosoma are twelve free segments, plus the tail-spine, of which the 

 first six form the mesosoma, and the remainder the metasoma. On the ventral 

 surface the segments of the mesosoma bear pairs of plate-like appendages, each of 

 which slightly overrides the next succeeding one. These appendages bear on their 

 inner (posterioi-) surfaces the lamellar branchiae, which are oval in outline (Fig. 



Enrypicrm fischerl Eichw. Silurian ; Island 

 iniliviilual, Vi (from Holm). 



of Oesel. Female 



