SECT. II 



THE OUTER BODY FORM 



being broad and flat. This form is due to a ridge run- 

 ning round the anterior surface as prolongation of the 

 lateral edges of the shell fold. This ridge is of consider- 

 able interest, as it appears in almost every Trilobite. 

 We shall later find reason to believe that it was a 



2 



--c 



FIG. 2. Diagram of head and first trunk segment of Apus, for comparison with 

 Fig. i the lettering the same. In addition : s, shell gland (the acicular gland 

 of Fig. i drawn into the shell fold). The distances between the limbs are much 

 exaggerated in order to show their forms more clearly. 



primary differentiation of the new " head," i.e. the new 

 Crustacean head, composed of five Annelidan segments. 

 Its origin and modifications will be discussed in 

 another place. This form of head is no doubt useful 

 for swimming and perhaps for burrowing in the mud. 



