40 THE APODID^: PARTI 



It is clear that this division of the under lip must have 

 been a very primitive feature in the Crustacean- 

 Annelid. It was absolutely necessary if the ventral 

 parapodia (posterior to the under lip) were to be used 

 for pushing food forwards into the mouth, which 

 habit led the way, according to our theory, in the 

 differentiations which transformed the Annelid into 

 the Crustacean. This early division of the under lip 

 accounts for its appearance as two ventral projections 

 in Limulus, where such a form can have no special 

 significance. The mouth in Limulus is a long median 

 slit, and instead of only the mandibles and two 

 maxillae being used as jaws, the ventral parapodia of 

 the four posterior cephalic and the first trunk limb 

 perform the masticator}' functions. 



We find a divided upper lip in some Trilobites (see 

 Fig. 49, p. 220), which offers a curious parallel to the 

 divided under lip of Apus and Limulus, but must 

 naturally be due to other causes (see however note 

 p. 241). 



IV. The First Maxilla. Unlike the mandibles, 

 the first maxilla has retained a small rudiment of 

 the dorsal parapodium in the form of a fold. That 

 this is, in fact, the reduced dorsal branch is clear 

 from a comparison of the musculature with that of the 

 other limbs (see Fig. 8 B). In the higher Crustacea, 

 this limb also may redevelop its dorsal branch as 

 maxillary feeler. The maxillai work behind the 

 lower lip. The limb itself requires no special descrip- 

 tion, its form can be seen from the figure. 



In Limulus the dorsal parapodium is well developed 



