CONTRIBUTIONS TO THE ANATOMY OF THE HIRUDINEA. 423 



and Vaillant^ for Pontobdella have endeavoured to show that 

 the true metamerism of the Leeches is expressed not by the 

 single annuli, but by the recurrence of cutaneous somites 

 consisting of groups of three to five annuli. 



My results are completely in accordance with such a view, 

 but I am able to show that there is a uniform law affecting the 

 whole group. 



The external evidences of metamerism are most readily 

 studied in Pontobdella, and although at first sight specimens 

 appear to differ very much, a closer examination shows a 

 universal uniformity. 



The papillae upon the surface may be very pronounced, conical 

 with a large base and branched apex, or they may be so far 

 drawn in as to leave the animal almost perfectly smooth, or 

 they may be in any condition intermediate between these two 

 extremes. 



The complete and remarkable change in the extent to which 

 the papillse are protruded may be easily observed upon living 

 specimens, and preserved specimens exhibit very pronounced 

 papillse, or are almost smooth, according as they were killed 

 suddenly or very gradually. The specimen figured (fig. 1.) 

 represents an average condition. 



Vaillant (1. c), who discusses the earlier observations of 

 Savigny, Leach, Baster, and Moquin-Tandon with regard to 

 this point, when speaking of the species he considers to be 

 P. verrucata, points out that taking, for example, the somite 

 following the clitellum (ceinture) it consists of four annuli. 

 The most anterior of these bears upon each side of the dorsal 

 median line two large papillse, while upon the three following 

 annuli the tubercles are less pronounced and so arranged that 

 the two papillse on either side of the median dorsal line upon 

 the second annulus are placed slightly nearer together than the 

 corresponding papillae upon the first annulus, those of the third 

 annulus farther apart than those of the second, while those of 

 the fourth annulus are even nearer the median dorsal line than 

 either of the foregoing. I have obtained three, perhaps four 

 1 ' Ann. Sci. Nat. Zool.,' 1870. 



