352 THE RECAPITULATION THEORY 



Of the two groups excluded above, the Crustacea 

 and the Vertebrata, the interest of the former 

 centres in the much discussed problem of the 

 significance of the Nauplius larva. There is now 

 a fairly general agreement that the primitive crus- 

 tacea were types akin to the phyllopods i.e., forms 

 with elongated and many-segmented bodies, and 

 a large number of pairs of similar appendages. If 

 this is correct, then the explanation of the Nauplius 

 stage must be afforded by the phyllopods them- 

 selves, and it is no use looking beyond this group 

 for it. A Nauplius larva occurs in other Crustacea 

 merely because they have inherited from their 

 phyllopod ancestors the tendency to develop such 

 a stage, and it is quite legitimate to hold that 

 higher crustaceans are descended from phyllopods, 

 and that the Nauplius represents in more or less 

 modified form an earlier ancestor of the phyllopods 

 themselves. 



As to the Nauplius itself the first thing to note 

 is that though an early larval form, it cannot be a 

 very primitive form, for it is already an unmis- 

 takable crustacean ; the absence of cilia, the forma - 

 tion of a cuticular investment, the presence of 

 jointed schizopodous limbs, together with other 

 anatomical characters, proving this point conclu- 

 sively. It follows therefore either that the earlier 

 and more primitive stages are entirely omitted in 

 the development of Crustacea, or else that the 

 Nauplius represents such an early ancestral stage 

 with crustacean characters which properly be- 

 long to a later stage, thrown back upon it and 



