PHYLÜM VII 



•80 



ARTHKOPODA 

 three regions of tlie body, and the appendages of the prosoma 



of Segments in the 



iff identical in nui ^ ., 



Tl e gcneral fonn of tl.e boly in Euryptends 



„f a Scorpiou, Imt ia relativoly broader and shorter. 



beingidentical in_numteandl.«<iUon.^^^^^^^^.^^^ ^^.^ ^^^^^ .^ somewliat like tliat 



■t# 



-^&^^' 



Tlie prosoma or cephalotliorax 

 consists of six fiised Seg- 

 ments covered by a quadrate 

 carapace witli its front 

 angles rounded. This bears 

 on its dorsal surface two 

 pairs of eyes— large kidney- 

 ^^haped lateral eyes, and 

 median ocelli. Tlie Com- 

 pound lateral eyes are 

 smootb in theEnrypteridae, 

 facetted in thePterygotidae. 

 As sliovvn by Clarke and 

 Rnedemann, tbese facetted 

 eyes are identical in struc- 

 ture with tbose of Limulus. 

 On tlie ventral surface 

 of tlie prosoma (Fig. 1506) 

 are seen six pairs of appen- 

 dages, of wliicb tbe 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 developed 

 as legs. These legs consist 

 typically of a basal Joint 

 {coxa), the inner margin 

 of which (gnathohase) is 

 provided with teeth and 

 able to function in masti- 

 cation, much as in Limulus 

 or Ä23US, 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 

 "epicoxite" 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 

 fii-st 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 (posterior) surfaces the lamellar branchiae, which are oval in outline (Fig. 



Fig. 1506. 



Eurypterus fischeri Eicliw. Silurian ; Island of Oesel. 

 individual, 1/2 (from Holm). 



Female 



