ZOOLOGICAL POSITION OF THE TRILOBITES. 45 



immediately following are, in the Chaetopods, often reduced 

 in size, progressively increasing till a maximum is reached. 

 This primitive annelidan characteristic is found in Triarthrus, 

 as well as in the related Limulus and Eurypterus ; the sixth 

 pair in the last two animals, as well as in Apus, being the 

 most powerfully developed. 



(d) Their hypothetical annelidan ancestor requires that 

 both the Trilobites and A pits originally possessed primitive 

 phyllopodan appendages. This has now been fully estab- 

 lished by Dr. Beecher's discovery that the rudimentary 

 limbs under the pygidium of two Trilobites, Triarthrus and 

 Trimicletts, were phyllopodan, like the larval and rudi- 

 mentary limbs of Apus. 



(e) Such phyllopodan limbs must have extended along 

 the whole length of the body as far as the metastoma. In 

 Apus the phyllopodan type persists as far forward as the 

 sixth segment, and can be faintly traced as far as the third ; 

 while Triarthrus teaches us, as we have seen (b), that the 

 homogeneity, of the limbs extended originally as far for- 

 ward as the second pair. We have thus a conclusive proof 

 that the common ancestor of both Apus and the Trilobites 

 possessed simple phyllopodan appendages along the whole 

 length of the body from the second segment. 



These are some of the new points relating to the ap- 

 pendages as a whole, which arise out of a comparison of 

 Dr. Beecher's discoveries with the known structure of Apus. 

 There are many other confirmatory points of interest and 

 importance which, if space allowed, might be adduced, but 

 these are the ones which I have selected to emphasise, be- 

 cause they demonstrate once for all the affinities of these 

 early Crustaceans to one another as common derivatives of 

 an annelidan ancestor, from which neither is far removed. 



Havingthus briefly traced the Trilobites and their existing 

 ally Apus to their common ancestral form, it remains for us to 

 indicate, as far as our knowledge goes, their respective diver- 

 gent specialisations. However interesting it is to run them 

 back to their common form, it is perhaps even more interest- 

 ing to trace the lines of advance leading them to their different 



