796 CLASS V. ARACHNIDA. 



same direction. The Eurypterida, though more primitive than 

 Limulus in the complete segmentation of the body, afford no 

 support to its Crustacean affinities ; on the contrary they present 

 a highly specialized feature in the fusion of the seventh pair of 

 limbs to form the metastoma. 



On turning to the air-breathing Arachnida we find, as was- 

 partially recognized long ago by Straus Durkheim, an almost 

 complete correspondence in the adult structure, and the in- 

 vestigation of the development of Limulus and of Scorpio has 

 brought to light further resemblance, and also cleared up points 

 of apparent differences. 



It appears that we have in Limulus a member of the Arachnida, 

 which retains the primitive water-breathing habit, and, in the 

 features of the abdominal appendages, some traces of the 

 characteristic structure of the far off crustacean stock from 

 which the Arachnida originally sprang. 



Order 2. EURYPTERIDA. 



Merostomata with the abdomen completely divided into twelve 

 segments, of which the five anterior bear appendages. A meta- 

 stoma is present, apparently representing the seventh cephalothoracic 

 limbs. 



The Eurypterida \vere aquatic (marine and freshwater) forms, 

 preserved in the Palaeozoic rocks from the Silurian to the 

 Carboniferous. In some respects they are intermediate between 

 Limulus (Xiphosura) and the Scorpions, notably in the complete 

 segmentation of the abdomen, and the more anterior position 

 of the mouth. The mouth cavity (i.e. the cavity lying outside 

 the mouth proper and between the bases of the second to the 

 sixth appendages) is closed behind and below by a large oval 

 plate, the metastoma, presenting a deeper or shallower notch 

 in the anterior border. It corresponds in position with the 

 chilaria of Limulus, and apparently represents these appendages 

 fused together. If this conclusion is correct the cephalothoracic 

 appendages correspond in number with those of Limulus. The 

 chelicerae* are chelate, and are short and included between the 

 second pair of appendages in Eurypterus, or long as in Pterygotus. 



* The presence of these appendages in Eurypterus, suspected by Laurie,. 



