52 ALiSTKALIAN HIRUDINEA, V., 



From a study of the nervous system it is evident that 34 

 somites are represented potentially in the body of all members of 

 the Hiradinea, the number of these visible on external examina- 

 tion of the leech being always much less. In the ventral 

 nerve-chain lie 21 distinct, norn)al, ganglionic masses, and in 

 addition at either extiemity lies a much swollen ganglionic mass. 

 These latter terminal aggregations of the ventral chain represent 

 the fused ganglia of 13 somites, of which those represented in 

 the anterior terminal mass are represented externally as "abbre- 

 viated" somites at the anterior extremity of the body. The 

 somites denoted by the capsules of the posterior terminal gangli- 

 onic mass are not represented externally on the body as segments. 

 From this we can conclude that the missing somites of the 

 posterior extremity are represented in all the various groups of 

 the Hirudinea by the posterior sucker; and, furthermore, that in 

 the Ichthyohdellidce a number of tlie anterior somites are repre- 

 sented in the "capula." In support of this it may be stated 

 that the posterior sucker often shows a faint annulation, and the 

 same is often to be noted in connection with the capula of the 

 Ichthyohdellidfp.. Again, it is only by this means that we can 

 make the position of the genital npertures in the Ichthyobdellidm 

 coincide within somite-limits with that found in the Glossi- 

 phoniidoi, GnathohdelUdce, and Ileiyohdellidce, inasmuch as the 

 genital apertures in the IchtJiyohdellidai are more anteriorly 

 situated, if one leckons from the first visible annulus behind the 

 capula. 



If we are to regard the Hirudinea as having been descended 

 from an Oligochaetan stock, we must regard the ancestral form 

 as a uniannulate worm whose body consisted of 34 distinct 

 somites. More will be said in r-eference to this after the descrip- 

 tion of the somitic constitution of a number of forms which I 

 have had the opportunity of examining. 



In a study of various genera in which the number of annnli 

 entering into the constitution of a typical somite is different, one 

 has some opportunity of deciding definitely the order of origin of 



