44 SCIENCE PROGRESS. 



the mouth by the appendages. In order to perfect this 

 method of feeding, this ridge would have to be 

 modified. In Apus we find, in fact, that it has almost 

 entirely disappeared in the middle line, while its lateral 

 ends have developed into two fleshy projections looking 

 somewhat like jaws, and often mistaken for such. Thus 

 modified, the metastoma, on the one hand, ceases to be 

 a hindrance against the travelling forward of food along 

 the middle line into the mouth, while, on the other hand, 

 its lateral lobes help to prevent the escape of food on either 

 side when the mouth is reached. Now the metastoma of 

 the Trilobite Tviartkrus is especially interesting because it 

 still shows the primitive ridge right across the ventral surface, 

 not yet smoothed down in the middle line, while the two 

 ends are only beginning to form their lateral lobes. The 

 discovery of the metastoma of Triarthrus is thus another 

 confirmation, this time not only of the relationship between 

 Apus and the Trilobites, but also of their common derivation 

 from an Annelid modified in the manner described. 



Turning now to the limbs, they offer so many striking- 

 confirmations of the same conclusions that they alone are, 

 it seems to me, sufficient to place the relationships here set 

 forth beyond further question. We will briefly note some 

 of the leading points. 



(a) Our deduction of both Apus and the Trilobites from 

 a common annelidan ancestor, from which neither was very 

 far removed, requires that all the appendages except the 

 first pair should have been behind the metastoma. This 

 primitive condition, lost in Apus by the metastomial lobes 

 stretching behind the third pair of limbs, is apparently found 

 in Triarthrus. 



{[)) The series of appendages behind the metastoma 

 should have been structurally alike, that is, there was no 

 original specialisation of those limbs nearest the mouth 

 into mandibles and maxilla; as distinct from locomotory 

 appendages. This primitive condition is retained by 

 Triarthrus. 



(c) While the first pair of appendages is reduced to 

 a pair of cirri, one on each side of the labrum, the pairs 



