SECT. XII RELATION OF APUS TO CRUSTACEA 185 



ascertain here the general principles on which Limulus 

 has been modified. 



On comparing the external form of Limulus with 

 that of Apus, we find that the head region, while 

 retaining essentially the same form, is yet far 

 larger in proportion to the size of the body in the 

 former than in the latter. The eyes are further 

 apart, and two ocelli take the place of the unpaired 

 " eye " or directive sensory body. The dorsal shield 

 does not stand out from the body as a fold like that 

 of Apus. Its frontal and lateral edges are produced 

 downwards and outwards, so that the anterior part of 

 the animal proper is raised from the ground, and, 

 under the shield, has room for the movement of its 

 limbs. ^ The dorsal shield, in fact, forms a sort of roof 

 under wdiich the animal lives. It is as if the head 

 and anterior trunk segments of an Apus had been 

 pushed upwards and backwards under the shield, 

 being somewhat squeezed up in the process. The 

 whole life of Limulus is passed under cover, every 

 possible entrance being shut, or carefully guarded, 

 against enemies. 



The fusing of the head with the dorsal terga of a 

 certain number of trunk segments naturally deprives 

 these segments of any power of movement one upon 

 another. The abdomen is also quite rigid, although 

 in its embryonic state it shows distinct external traces 



^ This is not shown in the section of Limulus rotundicauda, Fig. 42, 

 which, being a median longitudinal section, passes through the forehead. 

 The vaulted shape of ihe shell is best shown in transverse sections, cf. 

 the sections of a Trilobite Fig. 54, p. 230. 



