94 THE ANCESTRY OF VERTEBRATES 



Head of Arthropods. — Just as in Vertebrates, the head of 

 Arthropods is composed of a terminal part, the acron, which 

 \s to be compared with the prostomium of Annelids, and 

 -a number of segments fused with it. As with Verte- 

 brates, opinions differ as to the number of segments incor- 

 porated into the head and as to the question, which is to 

 be considered as the first segment. HEIDER (1914, p. 504) 

 comes in this respect to conclusions diverging from those 

 of Goodrich (1897, p. 247) who first set about the question 

 in a general way. Probably others after HEIDER will arrive 

 again at other conclusions in which still less rudimeniary 

 • or intercalary segments without corresponding extremities 

 are to be assumed, and in which still more supposed 

 segments will prove to belong to the prostomium. More than 

 one pair of ganglia have fused to form the brain in the 

 prostomium of Annelids (cf. p. 48) and the same is pro- 

 bably the case in Arthropods. These ganglia, however, have not 

 the value of segmental ganglia and do not represent as many 

 segments. Thus the last word concerning the segmentation 

 of the Arthropod as well as of the Vertebrate head has no 

 doubt not yet been said, but it seems to me to be a 

 valuable result of our deductions, and a firm base for further 

 researches, that we have recognized that in the Vertebrate 

 as well as in the Arthropod head a praeoral lobe (prosto- 

 mium, acron) must be distinguished from a number of 

 segments. 



Primitive features of the branchial region. — In several 

 respects the branchial region of the Vertebrate soma seems 

 to exhibit more primitive features than the segments of 

 the trunk, though on closer examination it often appears 

 doubtful whether we have not to do with secondary modifi- 

 cations. In this respect may be mentioned: 



1. The dorsal nerve-roots are of mixed character, being 

 both sensory and motor. BALFOUR (1878, p. 193) tried 

 to account for this peculiarity by the assumption: "that 

 primitively the cranio-spinal nerves of Vertebrates were 

 nerves of mixed function with one root only, and that root 

 a dorsal one, and that the present anterior or ventral root 

 is a secondary acquisition". This deduction was based on 

 the vievv that, in the head, only dorsal but no ventral roots 

 are found, while in Amphioxus also no ventral roots had 

 as yet been discovered. As we shall see further on there 

 is every reason to consider the "occipital nerves" as ventral 



