SECT. XII RELATION OF APUS TO CRUSTACEA 179 



most plastic ; the slightest alteration in habit of 

 life, and every change in size and form of the 

 body, bring about some corresponding change in the 

 limbs. So that while, on the whole, stability of 

 type Is wonderfully exemplified in the Crustacean 

 limbs, too much weight must not be laid upon it, 

 since the same class supplies us with equally won- 

 derful examples of extreme plasticity. Specialisation 

 for some particular habit of life leads often enough 

 to modification which altogether obliterates the type. 

 It is not safe, then, to conclude, because the limbs 

 of a Crustacean do not now show the typical form, 

 that there is no way of connecting them with 

 typical limbs. We thus maintain that the assumed 

 failure of Limulus to show the typical Crustacean 

 or Phyllopodan limbs ought not for a moment to 

 weigh against the positive likeness between it and 

 Apus. 



Further, while Apus has, as we have seen, retained 

 the more primitive form of limb, not far removed from 

 the Annelidan parapodium, the manner of life of 

 Limulus has led to a specialisation of its limbs, but 

 not, it is important to note, to such extreme 

 specialisation that no points of resemblance with 

 the limbs of Apus are retained. On the contrary, 

 the likeness, in some respects, is so great that one 

 might almost be tempted to leave the limbs out of 

 account in the question of relationship ; they speak 

 equally strongly both ways. When we come to 

 compare the animals in detail, Apus having supplied 

 us with the clue, it will not be difficult to deduce 



N 2 



