BY E J. GODDARD. 75 



either by the fact that they never did spread to any distance in 

 an easterly direction, or that this hypothetical connection with 

 New Zealand is of enormous antiquity. 



In discussing the relationship of the two genera, Philcemon and 

 'Geohdella, it is interesting to see that they show marked affinities, 

 which in themselves are unique characteristics of the two genera, 

 viz., the presence of only two jaws, and the same position of the 

 eyes. These affinities must be seriously considered as representing 

 certain fundamental characters common to both, and probably to 

 be found in an ancestor common to both, inasmuch as one of these 

 points, viz., that of the jaws, is a most important factor to be 

 considered in connection with classification. At the same time 

 there are wide differences V)etween them which would seem to 

 indicate that both forms have long been differentiated sufficiently 

 for the generation of separate genera. I have pointed out, in 

 connection with the subject of metamerism, in another chapter 

 that in Geohdella the pentannulate somite has been derived from 

 the uniannulate segment by the addition of fourannuli posterior 

 to the primitive ring, whereas in Philcemon the sensory ring is 

 denoted by the third annnlus of the tetrannulate somite, indicat- 

 ing tiiat the order of origin of the annuli is quite different. 

 Tlie question is now to be considered whether this tetrannulate 

 condition has been arrived at by the absorption of the last annulus 

 of tlie pentannulate somite, as seen in Geohdella (or the pent- 

 annulate somite by the addition of another annulus to the 

 tetrannulate somite of Philcemon); or whether these two forms 

 were differentiated after the common ancestor had developed the 

 biannulate somite. One finds that, in connection with somite-con- 

 stitution, the chief change is that of extension, or in other words, 

 the generation of the nmltiannulate condition. This we know 

 definitely lias taken place extensively in all members of the 

 Hirudinea to a greater or less extent, but, at the same time, 

 there is no substantial scientific support behind the denial that 

 retrogressive changes ever take place, that is, that an abbrevi- 

 ation may take place secondarily. If one removes the last 

 annulus of the pentannulate somite of Geohdella, it will be seen 

 -that the sensory annulus would not occupy the same position as 



