SECT, xiv THE EURYPTERID^E 241 



ments, and one pair of springing limbs for short 

 sudden dartings backwards. 



It is clear, then, that the possession of these large 

 labra was attended with certain disadvantages in 

 feeding. It is therefore not improbable that some 

 of these primitive Crustaceans should show various 

 modifications. Smaller upper lips being an advantage, 

 the labra might almost disappear, so that the opening 

 of the mouth would be ready for its prey as soon as 

 it came in a line with it. 1 A natural concomitant 

 change in the under lips would also take place ; 

 they would develop into the large metastomata found 

 in the Euryptcridae, which clearly helped to prevent 

 prey slipping past the mouth as the animal darted 

 forwards. The more rapid the forward dart after 

 prey, the larger should the metastoma be ; otherwise 

 prey once shot over would be almost sure to escape 

 before the animal could turn round ; the animals 

 have no trap-like arrangement of trunk limbs in 

 which prey could be caught. We do not, it is true, 

 find from comparing Figs. 55 and 56 that the larger 

 rowing limb is accompanied by the larger lower lip, 

 still we think the above reasoning to be correct, and 

 that other factors, such as the higher development of 

 the sensory organs, compensate in this case for the 



1 On p. 40 we discussed the origin of the division in the under lip of 

 Apus so that it should not form a barrier to the pushing of food forwards 

 into the mouth. We now see that the divided upper lip of some Trilobites 

 there referred to (and well illustrated Fig. 49, p. 220), was also probably 

 intended to shorten the way into the mouth, only in this case round the 

 labrum from in front. The thi'ee small pairs of posterior cephalic limbs 

 may have assisted in this latter process. 



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