474 



2. Note on Oka's biannulate Leech. 



By J. Percy Moore, Philadelphia. 



(With l.fig.) 



eingeg. 13. Juli 1900. 



Any consideration of the Ichthyobdellid somite was intentionally 

 omitted from my recent paper (1900) on Microbdella as it was desired 

 to reserve the material relating to this family for a later occasion, but 

 the failure to mention Oka's earlier discovery of a biannulate leech 

 was a pure oversight for which I wish to make amends. 



The leech described by Oka (1895) is a species of Ozobranchus 

 which is doubtfully referred to 0. Menatesi (Blainville) Quatrefages. 

 Several species of this genus have been described but they are not yet 

 clearly defined. Usually the complete somite is 3-annulate or 4-annu- 

 late, but Oka's species is remarkable in having 22 ' somites each com- 

 posed of two rings only, the rest being, when differentiated at all, 

 uniannulate. In the anterior region of the body the annuii are of 

 approximately equal width, but posteriorly they are alternately wide 

 and narrow. As interpreted by Oka each somite in the posterior region 

 embraces a larger annulus followed by a smaller one. Indeed, no other 

 interpretation seems reasonable , for on the ventral side the intra- 

 metameric furrows disappear and thus each narrow annulus is asso- 

 ciated structurally with the immediately preceding wide one. 



The discovery of such a type among the Ichthyobdellids is of great 

 interest as furnishing another factor in the series of somite paralle- 

 lisms between the two principal phyletic lines of the Hirudinea. 



The remarkable similarity between the somites of this species 

 and the Glossiphonid Microbdella is indicated in the accompanying 

 figures, a somite of Pontobdella being added for comparison as an 

 example of a usually 3-annulate Ichthyobdellid. In each case the 

 somite is subdivided into a broad anterior and a narrow posterior an- 

 nulus by a furrow which disappears on the ventral surface. Oka does 

 not state the exact position of the ganglion but it probably lies, as in 

 other species of Ozobranchus , in the middle of the somite, that is, 

 chiefly in the posterior half of the large annulus, as in Microbdella. 

 The nephropores, as frequently happens in the Ichthyobdellidae , are 



1 O k a says 23 (II to XXIV inclusive) , but it is probable that each of the two 

 broad annuii which are regarded by him as belonging to somite II in reality con- 

 stitutes an entire somite. The one thus added would bring the full number of somites 

 exclusive of the sucker, up to 27, the number usually recognized in the correspon- 

 ding regions of other leeches. 



