510 Mr. H. M. Bernard on the 



from the other joints, all the segments in the typical Trilo- 

 bite are alike in form ; they diminish in size, however, pro- 

 gressively from front to back. Further, with regard to the 

 head-piece, it is not difficult to see that this has been built up 

 of a certain number of fused and thus modified segments. 

 The same is true of the tail-piece, as we gather not only from 

 its markings but also from the fact that some of the very 

 oldest Trilobites are found without any such specialized fusion 

 of segments. Characteristic of the Trilobite segment is the 

 pair of immense pleural folds, which can only be regarded as 

 secondary specializations. We conclude, then, that the 

 Trilobites must have been descended from some form made 

 up of simpler segments, i. e. of segments without pleurae, and 

 that these segments, with the exception of the first, containing 

 the mouth, and the last, containing the anus, were in all 

 respects similar in form, but diminished 'progressively from 

 the oral to the anal end. 



We look elsewhere, then, for more primitive segmentation 

 than that which we find in these ancient fossils. We have 

 not far to look, for there exists a group in which it attains its 

 greatest development, the primitive segments being apparently 

 but little altered by secondary specializations. I refer to the 

 segmented Worms. In typical forms we have here a long 

 string of segments, the first and the last alone being peculiar ; 

 all the rest, no matter how many, are structurally exact 

 repetitions of one another. Not only are the appendages and 

 markings of the segments faithfully repeated from segment to 

 segment, but if cut open the internal machinery of each 

 segment, excluding the reproductive, is found simply to 

 repeat the internal machinery of that which precedes it ; 

 muscles, nerves, blood-vessels, excretory organs, are all more 

 or less exactly repeated. 



That metamerism reaches its highest development in the 

 Worms there can then be no doubt; hence it is to animals 

 below the Worms in structural complexity that we must look 

 for its origin. Where segmentation or vestiges of it are 

 found in animals above the Worms — for instance, we have 

 traces of it in ourselves — we are justified, in the absence of 

 clear evidence to the contrary, in assuming that it has been 

 derived from Annelidan ancestors. It is to my mind almost 

 outside the bounds of probability that such a remarkable 

 repetition of parts could have been developed twice on quite 

 independent lines. 



Now the problem is, How did such an animal come into 

 being? Several interesting suggestions have been made, 



