50 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I42 



ganglia in the trunk pertains to a segment. A trunk segment, how- 

 ever, is determined by the somatic muscles, and the ganglia are 

 necessary to activate the muscles. In the blastocephalon, or the part 

 of the adult head derived from it, there are no somatic muscles. This 

 head region bears the eyes and antennae, and sense organs do not 

 form segments as do muscles ; but they also must have nerve centers. 

 Thus the claim that nerve ganglia define segments is not valid, ex- 

 cept theoretically, where there is no muscular segmentation. 



The preoral and intracerebral position of the ocular and antenna! 

 brain commissures appears to conflict with the claim that the ocular 

 and antennal ganglia belong to segments that were formerly postoral. 

 It is explained, however, that these commissures are formed after 

 the cephalization of the ganglia. Yet these ganglia, in common with 

 the other body ganglia, should have had free ventral commissures 

 before they were cephalized. If the cephalic lobe of the embryo is a 

 segmented region, it should have ventral ganglia corresponding with 

 its component segments, but the only ventral ganglion of this region 

 is the preoral frontal ganglion, which innervates the clypeal and 

 labral muscles and the ingrowth of the oral ectoderm that forms the 

 stomodaeum. This fact in itself should suggest that the embryonic 

 head lobe is a preoral anatomical unit. The connection of the frontal 

 ganglion with the tritocerebral ganglia does not make this ganglion a 

 tritocerebral element, since its circumoral connectives with the trito- 

 cerebral ganglia are equivalent to the connectives between any two 

 consecutive ganglia of the ventral nerve cord. 



Most of the theories of arthropod origins are based on the assump- 

 tion that the arthropods have been derived from polychaete worms. 

 Glaessner (1958) has described a fossil polychaete from the base of 

 the Cambrian, but the arthropods must have originated a long time 

 back in the Precambrian. It, therefore, does not follow that poly- 

 chaetes were yet in existence at the time when the arthropod pro- 

 genitors became differentiated from simple ancestral segmented 

 worms. What the arthropods and onychophorans may have in com- 

 mon with modern annelids, therefore, must be traced back to some 

 primitive common wormlike ancestor, which very probably was not a 

 polychaete or even a chaetopod. 



Until some embryo or some arthropod living or fossil is found 

 with a preoral segmentation, we have no real evidence that this part 

 of the animal ever was segmented. Theorists who put their faith in 

 a few small cavities in the preoral mesoderm have yet to prove that 

 these cavities ever belonged to true body segments. Since we shall 



