416 



COMPARATIVE ANATOMY 



CHAP. 



biramose. The anterior pair lies in front of the mouth, and in 

 Eurypterus is a small finely jointed pair of feelers, while in Pterygotus 



it is a pair of long chelate 

 limbs, with j)owerful pin- 

 cers. The basal joints 

 of the five subsequent 

 pairs of limbs are inserted 

 round the mouth, and 

 have masticatory ridges 

 directed inwards, which, 

 in Pterygotus, are specially 

 strongly developed on 

 the last pair. 



The last pair of ceph- 

 alic limbs is much more 

 strongly developed than 

 the rest ; they are oar- 

 shaped, and evidently 

 served as swimming feet. 

 On the under side of the 

 thorax of 6 segments are 

 found 5 plates (leaf-like 

 limbs) consisting of 2 

 lateral halves ; these 

 overlap like tiles, and 

 cover leaf-like gills. The 

 most anterior largest plate is called the operculum. The abdomen 

 is devoid of limbs. Behind the mouth is a large oval plate, the 

 metastoma. 



Fig. 283.— Pterygotus osiliensis. Upper Silurian. Under 

 side restored, and smaller than natural size (after F. Schmidt). 

 a, Epistoma ; me, metastoma ; 1-6, feet; 1, cUelicer; 6, rowing 

 foot with large masticatory ridge, 1:1 ; I-V, ventral plates ; 

 au, eye. 



The systematic position of the Gujantostraca is not clear. In the number and 

 position of the cephalic feet they agi'ee irith the Xiphosura. The leaf-like limbs of 

 the thorax also somewhat recall those of Limuliis, and above all the operculum in 

 the 2 gi-oups seems to be homologous. Their relation to the other Arthropoda, 

 especially to the Crustacea and the Scorpionidce, whom they resemble in appearance, 

 has often been pointed out, but nevertheless is not clear, because the morphological 

 .significance of the limbs, and especially of those which lie in front of the mouth, is 

 not known with anything like certainty. Our knowledge of the structure of the 

 Gigantostraca and Trilohites has during the last decade received such unexpected 

 additions through palffiontological investigations {e.g. discovery of the limbs of 

 Trilohites) that we may hope for further advance. 



Etirypterus, Pterygotus. 



III. The Hemiaspidse. 



These are extinct palaeozoic forms apparently related to the 

 Xiphosura and perhaps forming a sort of connecting link between them 

 and the Gigantostraca. 



The body falls into 3 regions : a head of considerable size covered 



