ARTHEOPODA — ARACHNIDS . 



87 



in the large specimens of Pterygotus, Slimonia (Fig. 41), and 

 Stylonurus placed on the wall. 

 The body is long, flattened 

 and segmented, and is divided 

 into two regions. The front 

 one consists of the head and 

 some body - segments fused 

 under a single shield. The 

 hinder region, or abdomen, 

 consists of twelve distinct 

 movable segments, the last of 

 which bears a movable tail- 

 plate or spine, the telson. On 

 the upper surface or back of 

 the front shield is a pair of 

 small simple eyes (ocelli) near 

 its centre, while a pair of 

 compound, facetted eyes is 

 placed on or near the front 

 margin. On the under surface 

 of the shield is the mouth, 

 with a plate in front of it 

 and another behind it (h in 

 Fig. 41), as in the trilobites. 

 In front of the mouth is a 

 pair of limbs, differing greatly 

 in size in different genera, but 

 always ending in pincer-claws 

 (chelae). At the sides of and 

 behind the mouth are five 

 pairs of limbs, variously modi- 

 fied in different genera for 

 crawling, swimming, or grasp- 

 ing, but always agreeing in 

 having the basal segment 

 toothed to serve as a jaw. 

 Thus there are six pairs of 

 limbs to the front region, 

 which is therefore supposed 

 to consist of six body-seg- 

 ments. On the under surface 

 of the first abdominal segment 

 are the openings of the repro- 

 ductive glands, covered by a 



Gallery 

 VIII. 



Between 

 Wall-cases 

 12, 13 & 14. 



Fig. 41. — Restoration of the under 

 surface of a Eurypterid, Sli77ionia 

 acuminata, by M. Laurie, b, 

 metastoma ; c, leaf-like gills seen 

 through the covering plates of 

 the mid-body ; g, telson ; i-vi, 

 appendages of the fore-body ; 

 vii-xviii, segments of the abdo- 

 men, VII and VIII covered by 

 the genital operculum, ix-xii 

 bearing gill-covers. Much smaller 

 than nature. (From Woods' 

 " Palaeontology," by permission 

 of the Cambridge University 

 Press.) 



