178 THE APODID^ part ii 



reasons why zoologists have not allowed themselves 

 to be influenced by the extraordinary morphological 

 likeness between the two animals, and why they 

 have maintained that this likeness is merely a 

 remarkable case of analogy. 



On the other hand, the morphology of Apus has 

 been such a perpetual puzzle, that its likeness to 

 Limulus, even combined with its acknowledged re- 

 tention of Annelidan characteristics, gave no key to its 

 systematic position, just as its likeness to the Nauplius 

 failed to suggest that it is itself the proto-Nauplius. 

 When once, however, we recognise the essentially 

 Annelidan and therefore primitive character of Apus, 

 and thus regard it as an archaic form, i.e., as a sur- 

 vival from early geological periods, its likeness to 

 Limulus takes on at once a new meaning. The diffi- 

 culties above mentioned deserve to be re-examined ; 

 fortunately they are not insuperable. Before going 

 into a detailed comparison of Limulus, let us briefly 

 indicate the way these three difficulties may be met. 



I. In the first place, the difficulty as to the difference 

 between the limbs of Limulus and Apus depends 

 entirely upon an exaggeration (a very natural ex- 

 aggeration) of the importance of limbs for the 

 purpose of classification ; we say, a very natural 

 exaggeration because, as above stated, it has been 

 by a close study of the homologies of the Crusta- 

 cean limbs that so much has been done to arrange 

 the Crustacea into natural groups. On the other 

 hand we ought not to lose sight of the fact that 

 of all organs of the body the appendages are the 



