228 THE APODID^E TART n 



jaws) and maxilla?. In Limulus we have five pairs 

 of nearly equally important jaws, on the four last 

 head, and first trunk, limbs. In the Trilobites we 

 find the mandibles, or chief jaws, between the 

 first trunk limbs, and masticatory ridges for pushing 

 the food into the mouth, as in Limulus, on the four 

 posterior head limbs. In the Eurypteridae \ve shall 

 find further combinations. We may perhaps find 

 in these different attempts to develop the best 

 arrangements of mouth parts almost as important a 

 factor in the development of the class of the Crustacea 

 as we think we have found in the development of the 

 shield. There can be no doubt that while it offered 

 some advantage to use the ventral parapodia of the 

 most powerful limbs as jaws, this must have been 

 attended by certain disadvantages. To this important 

 subject we shall return. 



. We repeat here what we said on p. 43, that we might 

 with some safety establish a rule that the closer the 

 forehead was pressed against the ground the less likely 

 would the antennae be to function as antennae ; they 

 might cither degenerate as they have done in Apus, 

 and, according to W r alcott's restoration, in the Trilo- 

 bites, or they might function as seizing organs or mouth 

 parts, as in Limulus. We shall have occasion later to 

 see the converse of this rule, and shall find that the 

 raising of the head leads not only to the further 

 development and pointing forwards of the antennae 

 as sensory organs, but also to the travelling of the 

 antennae themselves towards the anterior end of the 

 body, an advantage for the animals which has enabled 



