204 INVEETEBEATA chap. 



it is quite clear that the common ancestral stock of Crustacea passed 

 through a larval stage corresponding to the Nauplius. 



Now, on the principles laid down in the first chapter of this book, 

 we are driven to conclude that the Nauphus represents a common 

 ancestor of all Crustacea in however modified a form. Fritz Mliller, 

 in his work, Filr Darwin (1864), concluded that all Crustacea were 

 actually the descendants of a small oval, unsegmented species of 

 animal with three pairs of legs. Hatschek (1877 and 1878), on the 

 contrary, pointed out that such a conclusion implied that the Crustacea 

 had no af&nity with Annelida, nor with Peripatus, Arachnida, and 

 Insecta, in all of which the early embryo was comparatively long 

 and distinctly segmented, with a double series of coelomic cavities. 

 This conclusion Hatschek rightly thought to be incredible, and he 

 therefore adopted the opposite opinion, namely, that the Nauphus had 

 no ancestral significance at all, but that since in all Arthropoda — and 

 Annelida, for that matter — the segments were developed from before 

 backwards, so that the first was the oldest, there must in all exist a 

 stage in which there were only three segments in the embryo, and, 

 according to him, it was due entirely to a secondary modification that 

 the eggs of some Crustacea were hatched when they reached this stage. 



Korsehelt and Heider, agreeing in the main with Hatschek, 

 suggest that the Nauplius is an " Arthropodized Trochophore " — that 

 it represents the Trochophore of Annelida with certain Arthropodan 

 features precociously added. Balfour (1880), finally, whilst believing 

 that the Nauplius in its present form was much modified, yet believed 

 that it exhibited ancestral features, and that the hinder part of 

 the body had formerly exhibited a segmentation which it had 

 secondarily lost. / 



Amongst aU the views we have recounted, that of Balfour seems 

 to come nearest the truth. It was reserved for his pupil and 

 successor, Sedgwick, to enunciate clearly what Balfour instinctively 

 felt, viz. that the embryonic phase is secondarily developed out of 

 the larval stage, and not vice versa. Indeed, Hatschek's view is 

 thoroughly inconsistent with the fact that, when the larva does not 

 hatch out as a Nauplius, a cuticle is produced and shed by the embryo 

 whilst still within the egg-shell when it reaches the Nauplius stage, 

 thereby showing that formerly this stage must have been passed 

 through in the open, in the ancestors of the forms in which it is now 

 purely embryonic. 



We saw in Chapter I. that a larva, as compared with the 

 actual ancestor which it represented, is usually greatly diminished 

 in size, and that this diminution in size is not accompanied by a 

 representation of all the organs which the ancestor possessed, also 

 diminished to scale, because such diminution would render them 

 ineffective; on the contrary, we saw that those organs which were 

 functionally dominant in the ancestral condition are reproduced by 

 the larva, while the others are suppressed. 



We are, therefore, probably nearest the truth when we suppose 



