SECT. XIV THE EURYPTERID^ 249 



In bringing to a close these comparisons of the 

 fossil Crustacea with Apus and with our Crustacean- 

 Annelid, it may be interesting to see, set out in a 

 table, the various ways in which the parapodia in the 

 first six Annelidan segments have been developed — a 

 representation of the attempts of Nature to find the 

 best combination of head and mouth parts. 



The limbs used as jaws are in larger type, so that 

 the different masticatory arrangements may be seen 

 at a glance. 



A study of this table shows us that all the animals 

 which retained the early primitive arrangement of 

 crushing the food between the ventral parapodia of 

 the first trunk limbs, which were the strongest in the 

 body, have, with the exception of Limulus, died out. 

 It is not difficult to see that it is a great advantage to 

 have the mandibles as close to the opening of the 

 oesophagus as possible, otherwise the greater part of 

 the juices of the crushed animal would be lost before 

 it could reach its destination within the oesophagus of 

 its devourer. The enormous metastoma or under lip 

 of the Eurypteridse may have been partly an attempt 

 to avoid this loss. It does not seem improbable, 

 therefore, that the ultimate selection of the third pair 

 of ventral parapodia as mandibles may have assisted 

 in leading to the survival of the modern Crustacea. 

 On the other hand, the enormous growth of some of 

 these ancient forms (Pterygotus anglicus sometimes 

 being more than a metre in length) shows that they 

 did not apparently suffer from lack of nourishment on 

 account of the arrangement of their jaws. When, 



