Carpenter — Relationships between Classes of Arthropoda. 333 



"What is the history of their evolution as regards segmentation? 

 Olenellus is the oldest known genus ; and, according to Peach ('94), 0. 

 Kjendfi is its most primitive species. This Trilobite had sixteen body- 

 segments, in addition to the five-segmented head — only one more than 

 the typical -Malacostracan and Insectan number. And if we study the 

 segmentation of Trilobites generally, we find a slow but steady increase 

 in the number of segments from the Cambrian on to the dying-out of 

 the order in the Carboniferous. Taking from Zittel ('87) the genera 

 whose segmentation is clearly known, it is found that the average 

 number of body-segments present in the Trilobites of each great period 

 of the Primary Epoch work out as follows : — 



Period. 

 Cambrian 



No. 



of Genera. 

 12 



Average number of 

 body- segments. 



17-66 



Ordovician 





23 



18-58 



Silurian 





16 



19-34 



Devonian 





10 



20-70 



Carboniferous 





2 



20-75 



It is also noteworthy that in the Ordovician period, when the 

 Trilobites seem to have reached their culminating point, there lived 

 two genera with the largest number of trunk- segments actually known 

 (thirty -two). 



The history of the Trilobites suggests, therefore, that a steady 

 increase in the number of trunk-segments characterised the evolution 

 of that order; and, as we have seen, the most primitive Trilobite 

 known to us possessed only sixteen trunk-segments. It maybefaiidy 

 inferred from this that the ancestral Trilobites were by no means 

 " richly segmented " animals. It may also be inferred that the rich 

 segmentation of such a PhyUopod as Apus is at least as likely to be a 

 secondary as a primitive character, and that the most generalised 

 Crustacean we can imagine might have had no greater number of 

 segments than a modern lobster. 



What fuj'ther light can be thrown on the natm-e of the earliest 

 Crustacean type ? Grobben ('92) has suggested that the Entomo- 

 straca cannot be regarded as a primitive group to he set over against 

 the Malacostraca ; but that, fi'om a Phyllopod-like ancestry, the various 

 groups of the Entomostraca that have undergone either reduction in 

 their segmentation or degeneration, as well as the higher Malaco- 

 stracan orders, must be derived. But almost every student of the 

 Crustacea has seen in the Leptostraca something of a connecting-link 



