Structure of an Aust)'ali<in Land Leech. 217 



VI., like IV. and V,, may be regai'ded as consisting of three 

 annuli, the 10th, llth and 12th. 



The third ganglion lies in the 13th annulus, that is the first of 

 somite VII. This is the first unabbreviated segment. Behind 

 this are fifteen, each consisting of four annuli, the eighteenth 

 ganglion belonging to segment XXII., being situated in the 73rd 

 annulus. Altogether there are sixteen unabbreviated somites, 

 beginning with the 13th annulus and extending back to the 76th, 

 the last complete annulus. Reduction of the somites again 

 occurs and we find the nineteenth, twentieth, twenty-first, 

 twenty-second, and twenty-third ganglia are lodged in the 

 acetabulum ventrally and in front of the anus, the anterior ones 

 probably corresponding to the 77th, 78th and 79th annuli. The 

 first four of these are single ganglia and therefore represent 

 somites XXIII., XXIV, XXV., and XXVI. The twenty-third 

 or acetabular mass is, however, a fusion of seven ganglia as can 

 be easily seen either from sections or by examining the ganglion 

 with a lens. This means, therefore, that seven somites are here 

 represented, making in all XXXIII., which is the typical 

 number of somites for all leeches. 



The supraoesophageal ganglion lies in the 7th annulus and is 

 connected with the suboesophageal mass by nerve fibres passing 

 round the pharynx. This ganglion, developing as it does, quite 

 independently of the ventral nerve chain, does not represent a 

 segment but the prostomial lobe of the embryo. 



The relationships of the ventral ganglia, somites and annuli 

 are shown in the following table : — 



Ganglion. Somite. Anxlli. 



1. (Esophageal - - V. (I.-V.)- - - - - 7, 8, 9, (1-9) 



2. ----- VI. ----- - 10-12 



3. VII. (unabbreviated) - ^13-16 



4. ----., VIII. „ - - 17^20 

 . 5. ----- IX. „ - - 21-24 



6. ----- X. - „ - - - 25-28 



.7. ----- XI. „ - - 29-32 



' 8. ----- XII. „ - . - 33-36 , . 



9. ----- XIII. „ - - 37-40 



10. - - - - - XIV. „ - - 41-44 



