280 
Head ‘of Arthropods.—Just as in Vertebrates, the head of 
Arthropods is composed of a terminal part, the acron, which 
is 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 stijl less rudimentary 
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. 234) and the same is pro- 
bably the case in Arthropods. These gang!ia, 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 view that, in the head, only dorsal but no ventral roots 
are found, while in Amphioxus also no ventral roots had 
as vet been discovered. As we shall see further on there 
is every reason to consider the “occipital nerves” as ventral 
