SECT. XI DEVELOPMENT I49 



The fact that a Nauphus stage is passed through 

 by so many Crustaceans — by all, indeed, where the 

 larva is not hatched out at a higher stage of develop- 

 ment than the Nauplius, has received great attention. 

 It led the older naturalists to assume that the primi- 

 tive Crustacean must have been an animal like a 

 Nauplius. This view has, however, generally been 

 given up, on the ground that no such conclusion 

 can be drawn from a free-swimming larva which 

 is certainly much modified to suit its own special 

 mode of life as larva. The whole argument of this 

 book has, nevertheless, brought us back somewhat to 

 the old view, i.e. that the primitive Crustacean was 

 a Nauplius-like animal, viz., an Apus. At the same 

 time, the modern objections were largely justified, for 

 the Nauplius is only a larval form of the primitive 

 Crustacean, in some respects comparable with, but 

 much more advanced than, the Trochophoran larva 

 of the Annelids, showing, on the one hand, traces 

 of its adult organisation, and, on the other, modifi- 

 cations to suit its own special method of existence 

 as a free-swimming larva. There were no means 

 of deciding which features belonged to the adult 

 and which to the larva as such. The general like- 

 ness to Apus was never therefore understood to 

 point to the fact that the Nauplius was nothing more 

 nor less than an Apus larva, and that consequently 

 Apus was a primitive Crustacean. And yet there 

 seems to be no difficulty in this view ; indeed, had 

 it been put forward alone, it would, we think, have 

 met with some acceptance as a plausible specula- 



