426 ALFEED GIBBS BOdRNE. 



The single annulus of somite 5 lodges the fifth post-oral 

 nerve-ganglion {g n., 5) ; close to the male pore, i.e. in annulus 

 i of somite 6 (first clitellar) lies ganglion 6, while by the female 

 pore in a similar position lies ganglion 7, and in annulus i of 

 somite 8 lies as usual the next ganglion (8). In the posterior 

 region of the body the somites become much crowded together, 

 as also do the nerve-ganglia, and it is no longer possible to 

 correlate them. 



Counting the sub-oesophageal ganglion (as Vaillant does not) 

 there are twenty-three post-oral ganglia here as in all other 

 members of the group which I have examined. 



Branchellion. — Turning now to the consideration of this 

 genus and examining the somite following the clitellum, we 

 find the first annulus bearing so-called lateral or branchial 

 appendages which possess vascular dilatations ('' Mamelon,'' 

 De Quatrefages), while no vascular dilatation is present in those 

 of the succeeding two annuli ; these three annuli compose the 

 somite, the next following annulus bears appendages with 

 vascular dilatations. The existence of a system of vessels in 

 the most anterior annulus of a somite in Pontobdella, exactly 

 corresponding to the system of afiferent and efi'erent (cf. infra, 

 vascular system), vessels of these dilatations in Branchellion, 

 completely justifies the view of the homology of the somite 

 comprising three annuli in Branchellion with that comprising 

 four annuli in Pontobdella. 



Hirudo. — The external segmentation is not so well marked, 

 the annuli are not to be distinguished inter se to the extent 

 that they are in Pontobdella, but a comparison of Moquin- 

 Tandon's and Ebrard's figures of varieties, allowing in a few 

 cases for inaccuracies which as they had not given special atten- 

 tion to this point, might well have crept in, shows that a very 

 regular distribution of the coloured markings in metameric 

 repetition exist, the somite so indicated consisting of five 

 annuli (fig. 4). 



Every fifth annulus presents in all varieties which I have 

 examined a ring of white dots as pointed out by Gratiolet (1. c). 

 This is the first annulus of a somite. This annulus, as I have 



