ZOOLOGICAL POSITION OF THE TRILOBITBS. 35 



Phyllopods of anything like the pleurae of the Trilobites, and 

 the difference between the leaf-like limbs of the former and 

 the filamentous limbs attributed to the latter, were acknow- 

 ledged difficulties against the former supposition. While 

 on the other hand, the fixed segmentation of the Isopods 

 and their uniform division into well-defined head, thoracic, 

 and abdominal regions showed them to be lar more highly 

 organised than the Trilobites. Lastly, against both these 

 affinities the total absence of any traces of antennae in the 

 Trilobites served as a warning against any too rash assertion 

 of their Crustacean affinities. 



So far, however, as this absence of the antennae in the 

 Trilobites could tell us anything, the advocates of the Phyllo- 

 podan affinity were better off than the advocates of the 

 Isopodan affinity. In the Phyllopod Apus, the antennas are 

 not visible from the dorsal surface, being ventrally placed. 

 If the antennae in the Trilobites were like those of Apus, they 

 would have vanished with the rest of the ventral appendages. 

 Their absence in the Trilobites therefore is just what their 

 assumed relationship with Apus would lead us to expect. 

 On the other hand, the same fact was a serious difficulty 

 against the Isopod relationship, as these have very pro- 

 nounced, anteriorly-placed antennae, which made it difficult 

 to understand how they could be completely absent in the 

 Trilobites, if there were any genetic relationship between 

 them. 



In 1892 the present writer made a detailed study of 

 the morphology of the Phyllopod Apus, which led him to the 

 conclusion that it was the most primitive of all existing 

 Crustacea, and was, indeed, an ideal transition form between 

 the chaetopod Annelids and the higher Crustacea. Such a 

 •conclusion necessarily led to a comparison between Apus and 

 the earliest known fossils which had any claim to be con- 

 sidered Crustacea, viz., the Trilobites. Several new points 

 of resemblance were discovered, of which one was of prime 

 morphological importance. The most startling feature in 

 the organisation of Apus was shown to occur also in the 

 Trilobites. 



The peculiarity referred to is the remarkable character 



