26 T. H. MORGAN. 



" For the Crustacea Dohrn rejects the Nauplius theory (of 

 Fritz Miiller and Claus) and adopts that of Hatcheck, who 

 believes the Crustacea to have descended from parents which had 

 the form of Phyllopods, just as these Phyllopods have in turn 

 descended from Annelids. From these same Annelids, according 

 to Dohrn, the Pycnogonids have come down. The number of 

 their segments was originally more numerous than we now see 

 them, the presence of a pair of rudimentary ganglia in connection 

 with the last pair of ventral ganglia allows us to add an eighth 

 pair of appendages, and, together with the first, all of these 

 appendages were originally homotypes. . . . They received diver- 

 ticula from the intestine . . . and each appendage enclosed 

 within itself a reproductive organ with a special genital opening. 

 (The so-called excretory organs of the palps and of the oviger- 

 ous legs are the rudimentary sexual organs of these appendages.) 

 The appendages were much shorter than we now see them, the 

 heart showed many openings, etc. The supposed ancestor that 

 Dohrn reconstructs might be very well compared to an Annelid. 



" We also notice that Dohrn persists in his opinion, published 

 in his work of 1879, that the Pycnogonids have a parentage 

 neither with the Arachnids nor with the Crustacea (they have 

 developed by the side of the last and altogether independently). 

 In this I am in accordance with Dohrn. I was struck in the 

 first place by the very general presence in the Pycnogonids of a 

 characteristic larval form (the Protonymphon), and its presence 

 suggested to me the idea of their descent from an ancestor 

 resembling somewhat a larva which took its place by the side of 

 the hypothetical ancestor of the Crustacea — the Nauplius — and 

 this by the side of a third (the ancestorof the Annelids), and that 

 all three groups would have descended from a common ancestor. 

 I had tried to obtain in this way an explanation of the affinities 

 of the three groups of animals (Annelids, Pycnogonids, Crustacea). 

 Dohrn, on the contrary, took into account the larval forms, but 

 constructed a phylogeny comparing together those animals having 

 a considerable number of segments. It is true I do not wish to 

 deny that such a method of looking at it has not just as good a 

 basis of reason, only it appears to me at present that it can be 

 defended just as little, and certainly no better than the larval 

 theory. 



