2o8 THE APODID^ part ii 



Returning to the subject of the shield, while in 

 Apus the dorsal integument of the fifth segment 

 developed a large shell fold, we see no need for 

 believing that in Limulus there was ever a dorsal 

 shield projecting backwards as a fold. When we 

 come to consider the Trilobites we shall find reason 

 to believe that the frontal ridge was in all these 

 animals older than the dorsal shield, and had a dif- 

 ferent origin, the dorsal shield itself being a later 

 development. In Limulus, as in many Trilobites, the 

 ridge round the front of the head is produced back- 

 wards on each side to form two horn-like processes. 

 But we reserve the further discussion of this most 

 interesting subject for the next section, where it will 

 be more in its place, as in the Trilobites almost every 

 possible variation of the same essential type of cepha- 

 lothoracic shield is found, for the defence of the 

 anterior bent, and therefore exposed, segments. We 

 shall then see some reasons for concluding that only 

 those primitive Crustacea which developed shields, i.e. 

 cither dorsal folds like the Apodidae, or bivalve shells 

 like the Ostracoda, survived, Limulus being probably 

 the only exception to this rule. In many modern 

 Crustacea, however, these shields have again second- 

 arily disappeared. 



