38 SCIENCE PROGRESS. 



which sometimes exhibits no traces of segmentation, was in 

 fact a secondary modification. 



But this was not all ; in addition to the visible posterior 

 tapering away of the segments, Walcott's 1 hypothetical 

 restoration of the under surface, based upon his own and 

 previous discoveries, showed the limbs diminishing in size 

 towards the posterior end of the body, as they do in Ap?is. 

 There was then no escape from the conclusion that the 

 Trilobites were, like Apus, derivatives of forms with still 

 richer segmentation than they themselves possessed, and 

 that they, like Apus, were shortening their bodies by the 

 fixation ot a number of the posterior segments in a rudi- 

 mentary or larval condition. When, therefore, we added 

 to this the striking resemblances between their head shields, 

 the similarity in form and position of the large labrum, the 

 presence in both of an intestinal tube bent sharply round 

 anteriorly and ventrally so as to open backwards above the 

 labrum, the association of Apus with the Trilobites ceased 

 to be a matter of speculation, and it was safe to predict that 

 when actual discovery revealed to us more details as to 

 Trilobite structure, organs would be found homologous with, 

 and probably closely resembling, those of Ap2ts. For in- 

 stance, a pair of minute cirrus-like antennae could safely be 

 assumed to have existed ventrally on each side of the 

 labrum of the Trilobites, and thus not visible from the 

 dorsal surface. 



There was only one objection which might perhaps have 

 been raised against this association of Apus with the Trilo- 

 bites, though hardly by a morphologist. The limbs of Apus 

 are phyllopodan, i.e., flat, leaf-like, swimming appendages, 

 while those of the Trilobites, as far as the evidence went, 

 were filamentous, and, according to Walcott's restoration, 

 filamentous even in the rudimentary pygidial segments. 

 This difficulty, by no means insuperable, has, as we shall 

 presently see, been completely disposed of by the recent 

 discoveries. 



The first here to be recorded is not one of the 



1 "The Trilobite: New and Old Evidence Relating to its Organisation," 

 Bull. Mus. Comp. Zool., vol. viii., 1880-81. 



