68 AUSTRALIAN HIRUDINEA, V. 



iiiajority of forms, however, as stated above, the anterior portion 

 of the somite is affected after the addition of the first posterior 

 annulus. Later divisions typically affect both extremities, or in 

 some cases may be restricted, at least for a time, to the anterior 

 extremity. Examples of the latter are to be found in those leeches 

 in which the adult somite is tetrannulate. After the formation 

 of terminal non-sensory annuli the sensory annulus is, as a rule) 

 not affected, but sometimes is as a result of the inability of the 

 terminal annuli to undergo further division. 



In discussing the question of metamerism I have attempted to 

 explain the nature of annulation in all the forms as the result of an 

 action of extension. There is no scientific support behind the 

 flat denial that " abbreviation " or fusion takes place. However, 

 if such a process does occur, it is quite secondary in importance 

 and by no means frequent in occurrence. 



EXPLANATION OF PLATE III. 



Figs. 1 A, IB, IC, — Diagrams showing the aiiangement of the annuli in the 

 genital somite of Orohdella Whitmani, 0. Ijimai and 0. octonaria 

 respectively, according to the scheme laid down in the text. 



Figs.2A, 2B, 2C, — Diagrams of the same, showing the annuli of the genital 

 segments arranged according to Whitman's scheme. 



Jigs. 3 A, 3B, 3C, — Diagrams showing the arrangement of the annuli in 

 Geobdella, Phikemon, and Fontobdella macrothela respectively. 



