464 Dr. C. Semper on the Identity in Type 



show that I have succeeded in the most surprising manner in 

 demonstrating that all those difficulties either do not exist or else 

 prove nothing, and at the same time in finding out such ex- 

 traordinarily extensive resemblances, both in the type of the 

 three classes of segmented animals and also in all their special 

 relations, that he only, in my opinion, is justified in rejecting 

 my views who believes himself able to arrive at morphological 

 laws through physiological relations. 



The opponents of my views do not agree in essential points. 

 On the one hand Baer says : — (1) ventrum and dorsum are ho- 

 mologous in Vertebrates and Articulates ; (2) therefore this is 

 not the case with the ventral cord and spinal cord, for the 

 latter has a dorsal and the former a ventral position ; (3) the 

 Articulates have no brain in the sense that the Vertebrates 

 have, for their dorsal oesophageal ganglion is only the anterior 

 end of their ventral ganglionic cord ; and (4) the Articulates 

 have only a singly symmetrical development, but the Ver- 

 tebrates a doubly symmetrical one. On the other hand, 

 Gegenbaur tacitly presupposes certain points, such as the 

 distinction in type, to be proved ; the arguments brought 

 forward by him against my views are as follows: — (1) the 

 position of the ventral cord (in agreement with Baer) ; 

 (2) the dorsal position of the supraoesophageal ganglion, which 

 is comparable to the brain and spinal cord of Vertebrates (at 

 variance with Baer) ; (3) the asserted connexion of the sense- 

 organs with the dorsal oesophageal ganglion in the Articulates ; 

 and (4) the dorsal origin of the latter out of a dorsally placed 

 medullary plate. 



I will begin with Gegenbaur's arguments. The sense- 

 organs (eyes and ears) are very frequently connected with the 

 ventral ganglia in Crustacea, Insects, and Annelids ; the third 

 argument of Gegenbaur is simply incorrect. The second, the 



the old attempt to parallel the shell-gland of the Daphnia with the seg- 

 mental organs of the worms, he says, " from this it might possibly be 

 attempted to derive the Arthropoda, or at least the Crustacea, from the 

 worms." 



Here, then, is no mention of the Vertebrates and Ascidians. If Dr. 

 Dohm would show me the place where he published the former of the 

 two propositions quoted above before I did, I should be ready to give up 

 to him the honour of having first suggested this idea, and to confess that 

 I had completely overlooked his notification of it. 



Among later observers, Leydig and Zaddach are the only ones whom I 

 have to thank for support in the old line of investigation on which I have 

 again recently entered ; what, besides their work, has been mentioned by 

 still living older investigators as to the affinities of the segmented animala 

 can be of no use to me, as it contains only repetitions of earlier statements, 

 was never followed up in a consistent manner, and was in great part wrong 

 in its execution. 



