216 THE APODID/R PART n 



two origins arc, however, clearly quite different ; we 

 shall find later that they help us greatly in under- 

 standing the striking difference between the Ostracoda 

 and the other Crustacea possessing bivalve shells. 



In the meanwhile the development of shells directly 

 from the primitive head-shield seems to have taken 

 place in two directions. 



(1) By the development of the posterior edge of a 

 neck-lobe, or dorsal fold of the fifth segment, at first 

 carrying the thorns, as shown in Fig. 48, and later 

 forming a covering for the back. 



(2) By the growth and folding down of the wings 

 or horns of the crescent-shaped head-shield against 

 the sides of the body. 



To these two shell formations w r e shall however 

 return in discussing the probable origins of the 

 modern Crustacea. We shall also have again to refer 

 to the importance of the formation of such shields 

 protecting the whole body, and to the advantages 

 which they offered over all the other variations of the 

 head-shield. 



We conclude then that the shield of Apus was not 

 the primitive formation ; the ground type was, we 

 think, the head-shield, every variety of which we find 

 in the Trilobites. Thus although, in Apus, we spoke 

 of the ridge round the head being the prolongation 

 of the lateral edges of the shield, strictly speaking the 

 ridge and the shield were two independent develop- 

 ments of the primitive Trilobitan head-shield, the 

 former starting forwards from the lateral projections 

 necessitated by the bending of the cylindrical body, 



