202 STUDIES IN EVOLUTION 



transverse groove, but as yet the body segments are undevel- 

 oped. After the separation of the head and pygidium the 

 thoracic segments are introduced successively between the 

 head and abdomen until the full number is reached, and 

 the animal measures from 10 to 55 millimetres in length. 

 The segmented stages have been described fully by Walcott, * 

 and an outline figure of the stage with one thoracic segment 

 is given in figure 2. 



The final conclusions to be reached from a complete study 

 of the development and structure of these animals can as yet 

 be only surmised. It is quite evident, however, that they 

 are related to the true Crustacea. The Trilobita are shown 

 to be a primitive type in (1) their multiple segmentation, (2) 

 the irregular number of thoracic legs, and (3) the biramous 

 structure of the legs. They therefore present characters 

 common to the Entomostraca and Malacostraca. 



* Trans. Albany Inst,, X. 



