^S AUSTRALIAN HIKUDINEA, V., 



the same way in 0. Wliitmaul, thus proving that another annulus 

 has Keen achied in front of the sensory annulus, and this coukl 

 have taken place only hy division of annulus 1, and not of 

 annulus 2; that another annulus has been added at the posterior 

 region, and only annulus 4 could have been concerned in this. 

 In considering the passage now from the somite of six annuli to 

 that of eight annuli we find, as shown above, that the sensory 

 annulus has divided, and that no further annulus has been added 

 posteriorly. The only point to consider now is whether la, lb, 

 or 2 lias divided to give rise to the extra annulus anterior to the 

 ganglion. As the sensory ring is usually so stable, but has been 

 proved to divide in this case, it would seem very probable that a 

 division has taken place in 2 also. The order of appearance of 

 these annuli would then appear to be as shown in the diagrams 

 illustrating the constitution of these species. (Plate iii., figs. lA, 

 IB, IC.) 



Pontobdella macrotltela Schmarda. — The specimen of this 

 species which I had the opportunity of examining, offered special 

 interest for the study of the somite in that genus, inasmuch as 

 the limits of the somite are verj' clearly shown on external 

 examination without entailing any reference to papillae, etc. 

 In this species the somite is triannulate, and the annuli con- 

 stituting the somite consist of one very wide, and two equally 

 small. The annuli are found to be arranged so that the large 

 ring is very intimately bound up with a small one anterior to it, 

 and another posteriorly situated. The line of division between 

 any two small annuli is denoted by a very strongly marked 

 groove so clearly that, on a very casual glance, one could easily 

 determine the limits of a somite without reference to papill;t\ 

 The annuli are all provided with prominences, but, in the case of 

 the large annuli, the arrangement and importance of develop- 

 ment are quite ditierent. It is this large annulus which corres- 

 ponds to the large papuliferous annuli of other species of Fontoh- 

 delJa, such as P. aust raliensis and P. muricafa. The only abbre- 

 viated somites to be noted occur towards the posterior extremity, 

 and here it is found that a biannulate somite is represented by a 



