J. D. Tothill — The Ancestry of Insects. 



379 



(1900), working with the tiny embryo of Anurida, supposes 

 there are seven segments ; the presence of the extra pair of 

 appendages described has not been verified, and seems to be 

 extremely doubtful. Janet (1899), arguing from adult anatomy, 

 supposes there are seven segments ; the evidence is not at all 

 convincing. In the diagram I have, therefore, indicated six 

 head segments. 



The rudiments of the maxillse in insects are characteristically 

 paired on each side and possibly indicate a biramous condition 



Fig. 4. 



Fig. 4. Germ band of Paratenodera sinensis, a Chinese inantid, showing 

 the rudiments of abdominal appendages, and the spiracles. (Original.) 



of all the ancestral appendages. The evidence, however, only 

 points directly to the biramous condition of the first and second 

 maxillae. 



The hypothetical ancestor being now visualized (fig. 3), the 

 next problem is to search for such an animal in the various 

 kindred groups of which we have knowledge. 



The Arachnoidea are by general consent considered to be 

 specialized in their own particular direction and may be passed 

 by in silence. 



One of the most interesting of all groups is that of the 

 Trilobites. In early Cambrian times they were already greatly 

 differentiated and dominated the life of the oceans, thus 

 occupying the ecological position that the fish do to-day. Some 

 were very small, some more than a foot long ; some lived in 

 deep waters, some in shallow ; some were bottom feeders and 

 others pelagic. In Ordovician times the group reached its 

 climax both in the number of species and in diversity of form. 



