214 THE APODID^ part ii 



head-shield play. We are inclined to think that the 

 formation of the ridge round the front was the primi- 

 tive variation, because of its great use as a belt-like 

 shield round the unprotected head of the browsing 

 animal, especially if it went hand in hand with the 

 thickening of the cuticle of the frontal surface. 

 The lateral processes and the frontal ridge thus 

 formed the primitive head-shield of this whole group 

 of Annelidan-Crustacea, and every form of shell- 

 covering may have been developed out of this 

 primitive shield. As a matter of fact we find almost 

 every possible variation of this ground form. The 

 cephalothoracic shield of Limulus is one form, 

 due to its fusing with the two anterior trunk seg- 

 ments. But by far the most important of all these 

 variations was the development of this head-shield 

 backwards over the trunk to form a cover such as 

 that of Apus. We have already described (p. 15) 

 the probable origin of this shell as a fold of the 

 tergum of the fifth segment developed to carry thorns 

 for the protection of the exposed dorsal surface, the 

 head being bent round ventrally, A Trilobite, Acid- 

 aspis Dufrenoyi (Fig. 48), shows us the neck-lobe 

 developed into the kind of thorn -carrying fold we 

 had imagined. Such a fold as that possessed by 

 Acidaspis, if a little wider and carrying more thorns, 

 could very easily develop backwards over the trunk 

 into a shell fold, such as that possessed by the Apo- 

 didse, the thorn-carrying function eventually giving 

 way to that of forming a cover for the dorsal surface. 

 But this is not the only form of shell which can be 



