i8o THE APODIDy^ PART ii 



the limbs of Limulus from the parapodia of our 

 Annelid, and to explain the transformations which 

 have taken place. 



The first difficulty as to the form and order of 

 the limbs is thus, Ave think, fairly satisfactorily met 

 for the present by the following four considera- 

 tions : — 



(i) That the possibility of homologising the limbs 

 with typical Crustacean limbs must not be too much 

 insisted upon, in the face of the well-known plas- 

 ticity of these organs. 



(2) That the limbs of Limulus are in many points 

 as strikingly like the limbs of Apus as, in other 

 respects, they are unlike. 



(3) That the modification of the Xiphosuran limb 

 out of the Phyllopodan or Annelidan is fairly easily 

 traceable to the manner of life of the animals. 



(4) As to the number of the limbs — our whole 

 theory makes the number of limbs or segments 

 developed of no real importance. The method of 

 the development of new segments is such that few 

 or many may be developed according to the needs 

 of the genus. 



II. Turning to the geological difficulties, w^e think 

 these of even less weight than those founded upon 

 the dissimilarity in the form, number, and order of 

 the appendages. The habit of life of Apus from 

 earliest times must have been such that it could be 

 very seldom preserved in a fossil state. It was 

 probably first shut off from the ocean in brackish 

 lagoons, and was gradually driven by the struggle for 



