380 



J. D. TothiU — The Ancestry of Insects. 



With the differentiation and multiplication of fish in the 

 Silurian epoch there was initiated the beginning of the end of 

 trilobites and the number of species fell away rapidly. There 

 was a slight expansion in the Devonian and as in the preceding 

 age there was a marked tendency toward the development of 

 (to us) curiously bizarre and ornamented forms. There fol- 

 lowed a rapid decline and the group became extinct with .the 



Fig. 5. 



Fig. 5. The trilobite Triarthrus beckii. (From Handlirsch after Beecher.) 



passing of Paleozoic time. More than 2000 species have been 

 described and this can represent but a fraction of the total. 



It is a source of surprise that a group showing such collective 

 vitality should have given rise to no terrestrial forms as have 

 the Annelida, Malacostraca, Arachnoidea, Gastropoda, and 

 Yertebrata. Handlirsch indeed takes exception to this view 

 and derives a number of groups directly or indirectly from 

 these ancient crustaceans. As insects are numbered among 

 these the similarities may be examined somewhat closely. 



In Triarthrus beckii (fig. 5), studied with such signal success 

 by Beecher, there are two regions, head and trunk. On the 

 twenty or so trunk segments are serially homologous biramous 

 jointed appendages. The head carries five pairs of append- 

 ages, thus indicating at least five segments. 



The trilobite head problem here suggests itself. Beecher 





