THE EYES OF THE EXTINCT EURYPTERIDA 135 



The Eyes of the Extinct Eurypterida 



The Eurypterida were extinct Arachnida. They were often of gigantic 

 size and they inhabited the sea in the Silurian and early Devonian periods. 

 Specimens of Pterygotus (Fig. 96) have been found which measure over 

 6 ft. in length. They agree with Limulus in many points, and more 

 especially in the possession of a pair of both median and lateral eyes. 



swimming 

 paddle 



Fig. 96. — Pterygotus osiliensis. Dorsal View, reduced. An Extinct 

 Arthropod, found in Upper Silurian Strata, and showing Paired Pits 

 in the Situation of the Paired Median Eyes of Limulus and Euscorpius. 



(From Wood's Palaeontology, after Schmidt.) 



These were situated in the dorsal shield or carapace in the same position 

 as in the king crab, and also in the same position as in the ancient mailed 

 fishes, the Ostracodermata. The resemblance of Limulus to the latter is 

 striking, but there are also important differences in the general structure 

 of the body and limbs, which will be discussed in Chapter 23, on the 

 geological evidence of the antiquity of the pineal organ. 



The structural resemblances between the Eurypterids and certain 

 living Arachnida, the Trilobites, and Limulus may, however, be alluded to 

 at this stage. It is obvious that in the enormous period of time which has 

 elapsed since the extinct Eurypterids and Trilobites existed, divergent 

 changes in structure will have been evolved. These will be most evident 

 in the adults of animals which have descended from the original stock and 

 there will be less difference in the larval forms ; secondly, although the 

 larval forms may be expected to resemble the ancestral stock more closely 

 than do the adult forms, we may also expect very considerable differences 



