SECT. II THE OUTER BODY FORM 21 



of such an exoskeleton. Perhaps the best way to make 

 the subject clear is to discuss and illustrate these 

 principles. 



(i) First and chiefly, the cuticle is thickened for 

 the protection of exposed parts. We find the cuticle 

 of the dorsal surface of the head, which, on the bending 

 round of the anterior segments, was left entirely un- 

 protected, and of the upper surface of the shield, 

 considerably thickened ; also that of the exposed 

 segments, i.e. of those segments which are not covered 

 by the dorsal shield. Underneath the shield, on the 

 contrary, the skin of the body is very thin and flexible, 

 though towards the posterior edge of the shield it 

 begins to thicken. We find the same principle in 

 Limulus and the Trilobites, where, under the protec- 

 tion of the thick shield, the cuticle of the ventral and 

 lateral parts remained soft and flexible. 



We also find certain parts of the body thickened 

 for protection against other parts. Thus the outer 

 edges of the under lips are thickened for protection 

 against the working of the powerful jaws and of the 

 first maxillae, between which two pairs of limbs they 

 are placed. 



(2) We find local thickenings to counteract the pulls 

 of the muscles, and this in two ways, (rt'.) There are 

 thickened areas such as the ventral and lateral parts 

 of the mandibles, to resist the almost rectangular pulls 

 of the mandibular muscles. In the higher Crustacea 

 such thickenings of the cuticle go hand in hand 

 with the concentration and physiological perfection 

 of the muscle bands, which, instead of being attached 



