54 AUSTKALIAN IIIKUDINEA, V., 



Ill view of the results obtained by tliese worivers, the opinion' 

 of present day students n;ust be that the sensory annulus denotes 

 the nii<ldle or potentially middle annulus of the somite. 



I have had occasion, in mapping out the soinitic constitution 

 of various Australasian representatives of the group, to test 

 Wliitman's and Castle's methods of determiuing somite-limits, 

 and the conclusions are interesting in that they show that con- 

 siderable variation takes place in various genera regarding the 

 manner of origin of the annuli; and that, while Castle's conclu- 

 sions are found to V)e correct in the greater number, in other 

 forms there is a yreat divergence from the conditions which 

 obtain in the formei'. From a study of these, the importance of 

 the somitic constitution as a generic character has i)een found to 

 depend on the manner of the origin of the annuli. 



LiiimobdeUa aitstj-alis. — This leech, iov all practical purposes 

 as far as this discussion is concerned, niay be considered as 

 belonging to the genus Hirudo, inasmuch as it has the same 

 number of annuli, same pentannulate somite, position for the eyes 

 and genital apertures, same number and position of nephridio- 

 pores, and the general auatoniy is very closely related. Con- 

 sequently, we can safely conclude that it has the same somitic 

 constitution throughout as in those forms which would fall within 

 the limits of the genus Hiriido in its more narrowed sense. In 

 this way we can derive a.ssistancein the study of the metamerism 

 of Limnohdella auairalis, inasmuch as we find metameric sense- 

 papillae very well developed in such forms as Hirudo inedicinaiis, 

 and one can most legitimately use the position of the.se structures 

 as if they were present in L. australis itself. What is now to 1)6 

 stated in comiection with the metamerism of L. australis applies 

 equally well to all species of the Hir^idinea which would IaW 

 within the limits of the diagnostic characters of Hirudo in its 

 wider sense; that is, all species which possess 102 annuli, and 

 have the eyes situated on annuli 1, 2, 3, 5, and 8, and the genital 

 apertures situated in annuli 30, 35, and 36 respectively, 



