66 AUSTRALIAN HIRUDINEA, V., 



generic differences as regards tlie nature of the somite of the 

 various leeches. It would seem that the order of the appearance 

 of the annuli in these tetrannulate genera is different from that 

 in such a pentannulate form as Hirudo, and furthetmore that 

 this is very possibly the reason that these forms aie tetrannulate, 

 etc., and not pentannulate. For example, in somite vii., of 

 Liianobdeila australis, or in fact of any species which falls within 

 the limits of the characters given for the old genus Hiriulo, we 

 find four annuli. It is the second and nob the third annulus in 

 the latter which represents the sensory ring. I liave already 

 shown that somite iv. is biannulate, and somites v. and vi. 

 triannulate in L. cntstralis, and that in the former somite the 

 anterior ring is sensory and both its component annuli partlj^ 

 fused. Again in somite v., the middle annulus is sensory and 

 the anterior ring is partly fused with this, but the postei'ior 

 annulus is distinctly differentiated. This shows clearly that the 

 posterior annulus is the first non-sensory annulus to appear, and 

 that the order of appearance of the various annuli of the somite 

 is as follows : — 



Annuli 12 3 4 5 



Older of development ... e c a b d 



As has been clearly shown in the case of species of Poutobdella, 

 and Ozobranchus branchiatus, the fourth annulus of the somite is 

 represented by the anterior annulus of the tetrannulate somite. 

 The order of appearance of the various annuli would then be as 

 follows : 



Annuli 12 3 4 



Order of development ... ... d c a b 



Inasmuch as this holds in the case of all the Hirudinea in which 

 the adult somite is tetrannulate, and the former order in the 

 case of most pentannulate forms, it seems reasonable to conclude 

 that the effect of this variation is reflected in the natux-e of the 

 adult somite. In connection with this question I might mention 

 that in HcenierUeria, we tind the somites triannulate dorsaliy but 

 pentannulate ventrally. This condition evidently obtains in all 



