248 THE APODID.K PART H 



in animals whose ordinary progression is slow and 

 deliberate. 



In our general account of the probable manner of 

 life of these animals we have described the change 

 which we think took place in the upper and lower 

 lips, the former almost disappearing, while the latter 

 develops into a large fold projecting anteriorly, and 

 bearing exactly the same relation to the masticatory 

 ridges of the first trunk limbs as the labrum of Apus 

 does to the mandibles, only pointing exactly in the 

 opposite direction. The position of this metastoma 

 corresponds exactly with that of the under lips of 

 Limulus. This fact seems to suggest that this wa^ 

 also the position of the under lips in the Trilobites. 



The leaf-shaped abdominal limbs we have already 

 mentioned as undoubted links between these animals, 

 Limulus, and our bent Annelid. 



We must now leave these highly interesting animals, 

 w r hich in point of size reached the highest develop- 

 ment of all the Crustacean descendants of our car- 

 nivorous Annelids. The exact relationship of the 

 group to the Trilobites and the Xiphosuridae, and to 

 one another, we cannot pretend to settle. It must be 

 left to those who have made the special morphology 

 of these fossil forms a life-long study. We must con- 

 fine ourselves here to the suggestion made above, that 

 the Xiphosuridse and Eurypteridae are early Trilobites 

 modified for two different and opposite methods of 

 feeding. We shall be more than satisfied if we have 

 been able to contribute something to our knowledge 

 of the groups, by tracing their origin to the Annelids. 



