470 Br. C. Semper on the Identity in Type 



argument it must be presupposed that the extremities of the 

 Crustacea &c. are homologous with those of the Vertebrates. 

 Rut this is bv no means the case. On the contrary, the 

 Annelids have dorsal appendages which stand in the same 

 relation to their dorsum as the extremities do to the ventrum 

 in the Vertebrates ; the dorsum of the former and the ventrum 

 of the latter, however, are, according to my view, identical. 

 In this case, therefore, one Avould have to compare the ex- 

 tremities of the Vertebrates to the dorsal feet, and the appen- 

 dages of the Arthropods to the ventral feet of the Annelids. 



Baer says further that the ventral side of the Annulates is 

 indicated as such by the ventral position of the anus and 

 genital openings. This, hoAvever, is only partially correct. 

 In the segmented Nemertines and in some Annelids the genital 

 apertures are dorsal ; in the Nematodes and IMyzostomidas the 

 efferent ducts of the sexnal organs unite, as in the Verte- 

 brates, with the rectum ; if they lie on the ventral side, they 

 undergo an unusual change in position. This variability in 

 the position of the genital openings shows that it is quite 

 valueless, because it is so extremely uncertain. Further, in 

 many Annelids (the leeches for example) the anus is situated 

 not ventrally but dorsally, and beyond it extends a prolon- 

 gation of the body (viz. the posterior sucker of the leech), 

 which, in its typical structure and in its origin, may be fairly 

 compared to the tail of the Vertebrates ; and one can then 

 designate the posterior ganglion of the leech as caudal ganglion. 



The only just argument brought forward by Baer is the 

 ventral position of the mouth in all the Annulates. But it is 

 a question whether the diflerence of its position in Annelids 

 and Vertebrates may not be satisfactorily explained. Dohrn 

 has made an attempt in this direction which is worth notice, 

 although others may be put by the side of his, for which it is 

 not necessary to enter into such bold speculations as Dohrn is 

 of course obliged to do. 



He rightly lays stress on the fact that the unusually late 

 appearance of the Vertebrate mouth is a very remarkable 

 circumstance. In distinction to this is the fact that the mouth 

 appears extremel}' early in all Annelids, in the free-swimming 

 larva} of the marine Annelids even earlier than the " Keim- 

 streif." That part of this last, through the segmentation of 

 which the cephalic portion of the worm arises, necessarily 

 finds an obstacle in the already developed gullet, and so curves 

 upwards around it in two divisions. The existence of the 

 gullet as a mechanical obstacle is the essential cause of the 

 formation of the oesophageal ring. 



In the Vertebrates, on the other liaud, the cephalic portion 



