432 MR. H. M. BERNARD ON THE [Aug. 1 894, 



dorsal edges, gills and sensory cirri, and distally specialized into 

 locomotory organs. The alimentary canal ran through the whole 

 length of the body, bending round anteriorly to open through the 

 mouth. 



The trilobites may thus be briefly described as fixed specialized 

 stages in the evolution of the Crustacea from an annelidan ancestor, 

 which bent its mouth round ventrally so as to use its parapodia as 

 jaws. 



Postscript on the Relation of the Isopods to the T?'ilobites. 



[The suggestion that the isopods are the modern representatives 

 of the trilobites must be judged on its own merits. The argument 

 in the foregoing paper is not in any way affected by it. The 

 relationship between Apus and the trilobites would remain intact, 

 the question being merely the following, " Can the isopods be 

 deduced directly from trilobites with five head-segments, that is, can 

 they be drawn from the main crustacean stem below Apus, or have 

 they branched off from the higher Crustacea above Apus? " The 

 former is practically the position taken up by MacLeay ' (referred to 

 by the President in the discussion which followed the reading of 

 the above paper). That able observer recognized the relationship 

 between Apus and the trilobites, but placed the latter between 

 Apus and the amphipods, probably without any clear notion of 

 what we now mean by descent. 



I am myself disposed to think that the isopods and amphipods 

 are but repetitions of the same process above Apus as that which is 

 illustrated by the trilobites below Apus. If the trilobites were 

 primitive Crustacea lower than Apus, specially adapted to a 

 creeping mode of life, the isopods may be crustaceans higher 

 than Aptis adapted to the same mode of life, and therefore closely 

 resembling the trilobites. The well-developed anteriorly-placed 

 antennae, the unmistakably crustacean mouth-formula, the sharp 

 division into thorax and abdomen, show the isopods to be Crustacea 

 above Ap>us. Hence I cannot help thinking that they are related 

 to the trilobites, not directly, but indirectly through Apus. — June, 

 1894.] 



Discussion. 



The President complimented the Author on the clear manner in 

 which he had shown the homologies between the ancestral form 

 Apus and the trilobita. He called attention to W. S. MacLeay's 

 ' Observations on Trilobites,' published in 1839, in which MacLeay 

 had proposed to place the trilobita between the entomostraca and 

 xiphosura on the one hand and the isopoda and amphipoda on the 

 other. He thought that MacLeay deserved credit for his acute insight 

 into the relations of these forms, and that, too, at a time when but 



1 W. S. MacLeay, ' Observations on Trilobites, founded on a Comparison of 

 their Structure with that of living Crustacea,' in Murchison's ' Silurian 

 System,' pt, ii. 1839, pp. 666-669. 



