1 86 THE APODID/E PART n 



of segmentation. The length and rigidity of the 

 caudal spine, taken together with the rigidity of the 

 anterior portion of the body, would render the flexi- 

 bility of the middle part of little use. It is true that 

 there are Trilobites with rigid shields and pygidia 

 and yet with flexible segments in the middle region 

 of the body ; but the pygidium could, in these animals, 

 be used as a covering in the rolling up of the body. 

 In animals possessing a caudal spine there could 

 be no question of its being put to any such use ; 

 hence, probably, the rigidity of the middle or ab- 

 dominal region of the body. 



There are, comparatively speaking, but a small 

 number of segments in the Xiphosuridae, at least as 

 compared with Apus ; but this is a matter of com- 

 paratively small importance if we take into con- 

 sideration the method of development of the early 

 Annelidan-Crustaceans. The hind part of the body 

 might become fixed at almost any stage of develop- 

 ment, more or fewer segments coming into existence 

 according to the degree of specialisation of each 

 group. Apus developed, comparatively speaking, a 

 large number (50-60), Limulus a small numbcr 

 (ca. 1 6). 



The metamerism of Limulus is probably to be 

 reckoned as follows : 



Cephalothorax. This is composed of five segments 

 of the bent Annelid forming the head, each segment 

 retaining its appendages, and of two trunk segments 

 bearing two pairs of limbs, the posterior pair forming 

 the operculum ; in all seven segments. 



