•2l6 Proceedings of tJte Royal Society of Victoria. 



Having ascertained the topographical relation of the external 

 rings to the nervous ganglia, these being taken on account of 

 their more regular raetameric arrangement, we may, by com- 

 parison with other leeches call the annulus in which the ganglion 

 lies the first annulus of the somite. The nephridiopores — -two — 

 open on the posterior edge of the annulus in front of the one 

 bearing the ganglion that is on the fourth or last annulus of each 

 segment. 



The number of somites is, of course, dependent on the number 

 of ganglia. By dissection it can easily be seen that as in all 

 Hirudinea, exclusive of the tiesophageal and acetabular nerve 

 masses, tliere are twenty one ventral ganglia, that is twenty-three 

 ventral ganglionic masses in addition to the supraa^sophageal 

 mass. Of these the first or suboesophageal mass lies in part of 

 the 8th and 9th annuli. By means of sections it can be seen to 

 consist of five fused ganglia. This is the case in most other 

 leeches. Comparing this with the structure of the unabbreviated 

 segments where we find one ganglion to every four atniuli, that 

 is to every segment, we can only suppose that the first nine 

 annuli containing the suboesophageal mass represent at least 

 five somites. 



These, if unabbreviated, ought to consist of twenty annuli. 

 Reduction of the rings, or, what Whitman calls " centripetal 

 abbreviation," has taken place, and the anterior somites have 

 merely kept those rings which are of the highest functional 

 value. By this means the first three segments may be regarded 

 as consisting of three almost indistinguishable rings bearing each 

 a pair of eyes. The next segment, IV., consists of the 4th, 5th, 

 and 6th rings, the first of these bearing the fourth pair of eyes. 



The eyes are undoubtedly modified and highly specialised 

 segmental papilUe and ought therefore to be on the first annulus 

 of each segment. 



Segment V. begins with the 7tli ahnulus bearing the fifth pair 

 of eyes and ha^, as in segment IV., l\!v'o other annuli, the 8th and 

 9th. " . ' 



In the next ring the 10th is the sebond ganglion, so that the 

 oe^bphageiir rnass may' easily be regarded as 'consisting of the 

 representative ganglia of the segments I.-V. Between the 

 second and third ganglia there are two rings, so that somite 



