•62 AUSTRALIAN HIKUDINEA, V., 



in botli, would seem to lend additional weight to the argument 

 that the gaps do signify the dividing lines of the somites. 



It is interesting to note that two of our Australian gen^-ra, 

 Semilagetieta and Geobdella, which are thus unique among the 

 Ilh'udinea, as far as we know at present, in that the somite- 

 -extension lias alfectod 011I3' the posterior region of the somite, tlius 

 leaving the sensory ring at the anterior extremity of the somite, 

 have also other unique characters which are of some note. As 

 mentioned previously, Seinilayeneta resetnl)les in many respects 

 the Glossiphoniidce, and again the Ichthyubdellidce. It is a 

 Rhynchobdellid leech, but does not find its place definitely 

 in either of the two divisions of that group. Again, Geobdella \^ 

 unique in that it possesses only two jaws and has the genital 

 apertures separated by seven and a half rings although the somite 

 is pentannulate. This latter peculiar character of Geobdella may 

 be due in some measure to the unique manner in which the 

 annuli have been generated in connection witli somite-extension. 



Another noteworthy feature about Semilageneta is that there 

 has been an absorption of somites at the anterior extremity, 

 analogous to the fusion of the anterior somites to form the capula 

 of the IcldhyobddlidK; and further, inasmuch as this absorption, 

 judging fi-om the position of the genital apertures, could only 

 have affected a few somites, tiiere must have been a great 

 absorption of somites at the posterior extremity, since the number 

 of somites represented externally is so very small in comparison 

 with other leeches. 



Semilageneta like Ozobranchns may yet have to be regarded as 

 a type of a new family intermediate between the IchUhyobdellidm 

 and the Glosniphoniidce. 



Conclusions. — -We must resard the llirudinea as havins" been 

 ^descended from an Oligochaetan-like worm which was uniannulate, 

 and whose body consisted of 34 distinct somites. This represented 

 the condition of the primitive ancestor of the grouj^, as it left the 

 main stem of the phylogenetic tree of tiie Annulata. Special 



