SECTION II 

 The Outer Body Form 



THE HEAD 



The comparative anatomy of the Crustacea has 

 long ago established the fact that the Crustacean head 

 must originally have been composed of five fused 

 segments of an annulate body. Our derivation of 

 Apus from a browsing Annelid explains the method 

 of this fusion, that it did not take place along the 

 longitudinal axis of the body, but by a doubling of 

 this number of segments upon themselves. This 

 term " doubling " or " doublature " has already been 

 applied to the under sides of the forehead of such 

 animals as Apus, Limulus, and the Trilobites, but 

 apparently meaning nothing more than the doubling 

 of the forehead, which has both a dorsal and a ventral 

 surface. In reality, however, this "doubling" is the 

 true description of the Crustacean head as shown in 

 Figs. I and 2 ; these should be further compared with 

 Fig. 46, p. 212, which represents a longitudinal section 

 of a Trilobite, where the doubling is very clear. 



