350 



Mr. A. G. Bourne. 



[June 21 r 



XII. " Contributions to the Anatomy of the Hirudinea." By 

 Alfred Gibbs Bourne, B.Sc. Lond., University Scholar in 

 Zoology, and Assistant in the Zoological Laboratory, 

 University College, London. Communicated by Dr. M. 

 Foster, Sec. R.S. Received June 21, 1883. 



(Abstract.) 



The author has investigated the following genera : — 

 Rhtncobdellidjj. — Pontobdella, Piscicola, Glepsine, Branchellion. 

 GNATHOBDELLmas. — Aulostoma, Hcemo/pis, Hirudo, Hcemadipsa, Ne- 

 phelis, Trocheta. 



The author gives a bibliography of the most important literature 

 upon the group since the appearance of Moquin-Tandon's monograph. 



External Characters and Evidences of Segmentation. 



The author, in attempting to answer the question — How far in the 

 series of Hirudinean genera do external characters express the meta- 

 merically segmented nature of their organisation ? — follows in the 

 footsteps of Gratiolet and Vaillant, who have dealt with Hirudo and 

 Pontobdella respectively in this connexion. The further question — 

 How far do such metameres represent the somites of a bristle-bearing 

 worm ? — first suggested itself to the mind of De Quatrefages. 



The author shows that these external evidences of metamerism in 

 Pontobdella are most complete, and further that they have a precise 

 relation to the metamerism expressed by the internal organisation. 

 The normal somite here presents four annuli of varying size, each 

 with its special and distinct arrangement of papillae. 



The clitellum involves two reduced somites, each consisting of two 

 annuli, the generative pores being placed between these respectively. 

 The nerve-cord exhibits a corresponding condensation in this region. 

 Twenty somites can be readily distinguished, while posteriorly there 

 are indications of several others, which is in accordance with the 

 existence of twenty- three post-oral ganglia and with Leuckart's obser- 

 vations {Hirudo) upon the condensation of an even greater number of 

 primitively separate ganglia in this region. They are rudiments of 

 originally existing somites. 



In Branchellion it may be shown that three annuli comprise the 

 somite, that in the median region, while every annulus bears a 

 branchia (lateral appendage), the most anterior annulus of the somite 

 bears a vascular dilatation at the base of its branchia. 



Similar dilatations, although in a more rudimentary condition, exist 

 in a similar position in Piscicola, Clepsine, and Pontobdella. 



