472 On the Identity in Type of the Annelids and Vertebrates. 



not arise on the dorsum ; a morphological distinction between 

 it and the ventral cord does not exist. 



5. In Annelids, Arthropods, and Vertebrates, cephalic may 

 be distinguished from body-segments ; in all cases the youngest 

 cephalic segment is next the oldest body-segment. 



6. In Annelids (Arthropods ?), as in Vertebrates, the type of 

 the collective organization is indicated by " evolutio bigemina " 

 (Von Baer). 



7. Beneath the nervous system of the Annelids lies a cel- 

 lular cord (chorda dorsalis?), indicating the axis from which 

 the two muscle-tubes extend round the alimentary canal and 

 central nervous system respectively. 



8. Beneath this cellular cord and above the alimentary canal 

 in the xVnnulates, there lies a vessel in which valves are entirely 

 wanting, and in which the blood flows from before backwards, 

 just as in the aorta of the Vertebrates. 



9. Tlie so-called dorsal vessel of the Annelids corresponds 

 to the Vertebrate heart ; it lies beneath the alimentary canal ; 

 and the blood in it flows from behind forwards. It is the sole 

 vessel which contains valves, and never loses its contractility ; 

 and it is always a venous heart, which last is the embryonic 

 type of heart in the Vertebrates. 



10. The external gills of the Annelids and Arthropods re- 

 ceive their venous blood, like those of the Vertebrates, direct 

 from the heart. 



11. The segmental organs of the Annelids appear on the 

 neural side, close beneath the axial cord and nervous system, 

 exactly as with the segmental organs of the Vertebrates. 

 (Hackel's section of the embryo of an earthworm is entirely 

 incorrect.) 



While, therefore, the hypothesis that ventrum and dorsum 

 are morphologically similar (homologous) regions in the Verte- 

 brates and Articulates has only the single morphological fact 

 of the ventral position of the mouth to support it, the view 

 that dorsum and ventrum are not similar in these animals is 

 based upon a whole series of the most important morphological 

 considerations. 



Quite apart from the correspondence resulting from this 

 view, in the vascular system, in the urogenital system, and in 

 the typical parts of the nervous system, three arguments appear 

 to me to be preeminently suited definitely to oppose the former 

 hypothesis. 



These are : — the proof that " evolutio bigemina " occurs also 

 in the Annelids ; the evidence that no distinction exists be- 

 tween the brain and ventral cord in the Articulates; and, lastly, 

 the facts, already mentioned by others, that in Annelids, Arthro- 



