51 



CONTRIBUTION TO A KNOWLEDGE OF AUSTRA- 

 LIAN HIRUBINKA. Part v. LEEcii-MKTAMERiSM. 



By E. J. GoDDAKD, B.A., H.Sc, Lixnean Macleay Felt.ow of 



THE Society in Zoology. 



(Plate iii.) 



The subject of '• Metamerism in the Hirudiiiea " has for a long 

 time attracted the attention of workers on the group. It has 

 long been known that the annuli which appear on the surface of 

 the leech do not represent true segments or metanieres, but that 

 in each genus more or less typically a certain number represent 

 conjointly the limits of a somite. The number of annuli thus 

 entering into the constitution of a somite is, as a rule, constant 

 fundamentally in each genus, although in connection with the 

 extension of the somite variations may take place in the various 

 species of a genus. This, however, happens in but few genera in 

 comparison with the number in which the numbfi- of annuli 

 entering into the formation of the " unabbreviated " somites is 

 constant, and characteristic of the genus. At either extremity 

 of the bod}' are found "abbreviated" somites, that is, segments 

 wliich do not comprise the full number of annuli as found enter- 

 ing into the formation of the complete somite which is present 

 in the middle region of the body. These " aljbreviated " somites, 

 when their exact limits have been carefully mapped out, throw 

 much light on the mode of formation and order of origin of the 

 annuli, as we |iass from the hypothetical primitive and uni- 

 annulate conditi n\ of the somite. For our present day concp.p- 

 tion of the limit of the somite we are indebted mainly to Castle 

 iind Moore. 



