64 AUSTRALIAN HIRUDINEA, V., 



Passing on from the uniannulate condition, we meet with 

 Microbdella which is biannulate in the adult state of the snniite. 

 In this case the second annulus has been added posterior to the 

 sensory annulus. By the addition of another annulus anterior 

 to these, we meet with the normal]}' developed triannulate adult 

 somite which is so prevalent among the members of the 

 Rhynckohdellidfe. 



Mesobdella, a distinctly Arhynchobdellid leech, rejjresents 

 the only member of that group, in which the somite retains its 

 simple triannulate nature. 



In some cases, however, \<'e find that the third annulus 

 developed is posterior to the second; in other words, that the 

 sensory annulus represents the tirst annulus of the somite; 

 examples of this are found in only a few genera such as 

 Semilagenetci and GeohdeUa. Such might well be expected as a 

 variation even in the development of the " fundamental " tri- 

 annulate somite, inasmuch as we tind considerable variation jn 

 the order of development of the extra annuli in the evolution of 

 the multiannulate somite. As far as I know, there is not one 

 known instance among the Ilirudinea in which the second annulus 

 of a somite is developed anterior to the sensory annulus. After 

 this annulus has been developed, it would seem then that two 

 lines allowing for variation are opened uj). In the majority of 

 cases we find the third annulus developed anteriorly, but in 

 some posterior to the second annulus. In the latter case, such as 

 in Semilagenefa and Geobdella, we tind the sensory annulus 

 represented by the tirst ring of the triannulate somite. The 

 stimulus given to the exclusive division of the posterior annulus 

 of the somite results in the formation, in Geobdella, of a 

 pentannulate somite, the most anterior annulus of which is the 

 sensory ring, and is to be regarded as representing potentially the 

 primitive annulus of tiie somite. 



In the following table is given a list, as complete as possible, 

 of the more important of the known genera of the Hirudinea, and 

 idicating the annulation of the various genera of the Ichthyol- 

 dellidce, Glossiphoaiida'; IIerj)obdeUidce, and Gnathobdellidm. 



