SEGMENTATION OF THE HEAD OF DIPLOPODA. 621 



In taking out the suboesopliageal ganglia from several 

 adult Spirostreptus I naturally looked at tlieir gnatlio- 

 chilaria. In two of these I found the hypostome to be dis- 

 tinctly double (text-fig. 6). This would seem to show that it 

 had its origin rather in two appendages than in one sternum. 



If, as above suggested, the gnathochilarium consists of but 

 one pair of appendages we have another reason in favour of 

 the homology proposed in the first part of this paper. 



Of making many homologies for the segments in different 

 arthropods there is no end, but most authorities are agreed 

 in assigning six segments to the head in Crustaceans, Hexa- 

 pods and Chilopods. Now the head of the adult Diplopod is 

 very distinctly marked off from the body, being separated 

 from it by a narrow neck, which forms, as it were, a joint. This 

 is more apparent in the recently hatched larva before the 

 chitin has developed than in the adult (fig. 3). Even in so 

 young an embryo as that shown in fig. 2 the ganglia of the 

 head have a different appearance from those of the body. 

 The Diplopod head can now be shown to consist of six 

 segments thus : 



(1) A segment which, together with the acron (Heymons), 

 forms the procephalic lobes. 



(2) The antennary segment. 



(3) The tritocerebral segment, representing the second 

 antennary segment of Crustacea, and tritocerebral rudiments 

 in Hexapoda and Chilopoda. 



(4) Mandibular. 



(5) First maxillary (rudimentary). 



(6) Second maxillary segment, the appendages of which 

 are fused to form the gnathochilarium. 



As the head of Spirostreptus is distinct from the body 

 so also, notwithstanding the forward movement of the 

 ganglia, is the suboesophageal ganglion distinct from that of 

 the post-maxillary segment. I mention this as further 

 evidence to show that the post-maxillary segment belongs to 

 the body and not to the head. 



