246 THE APODID^: PART II 



a head or a cephalothorax. Our homology of the large 

 limb with the first trunk limb, throughout all these 

 primitive Crustacea, shows that the six segments of 

 the Eurypteridae form a cephalothorax, and not only 

 a head. 



This degeneration of the anterior antennae in 

 Eurypterus is hardly what we should have expected 

 theoretically. The rapid forward movement for feed- 

 ing would seem to require highly developed antennae 

 pointing forwards. We attribute it to the fact that 

 the manner of life of the animal, as above described, 

 required that the sense of touch in a limb should be 

 immediately followed by an act of seizing, by means 

 of its masticatory ridges. The anterior antennae had, 

 however, entirely lost the power of developing their 

 parapodia even in the original Crustacean-Annelid, 

 and thus became of very secondary importance in the 

 life of Eurypterus. 



As to the other limbs of the head, Eurypterus 

 resembles Limulus in having the masticatory ridges 

 on the last four head limbs and the first trunk limb well 

 developed, and working as jaws round the mouth, which 

 was apparently not the case in Pterygotus, where the 

 importance of the masticatory ridges of the first trunk 

 limb over those of the head limbs was very evident. 

 The exact morphology of the limbs themselves it is im- 

 possible to describe with certainty; it is not improbable 

 that those of the head, in Eurypterus, are the sensory 

 cirri alone of the original parapodia. We sec no reason 

 why this should not be the case. Nature seems to 

 delight in every possible variation, and indeed in the 



