292 ASAJIRO OKA; OX SOME NEW 



the somites. By dissection, a ring that lodges a ganglion can easily 

 be determined, and starting from it we are able to find the boundaries 

 of the somites in a consideraloJe portion of the body, though at the two 

 extremities, it is almost impossible to group the rings into somites 

 appropriately. 



]^y this means, I found out that the three species above described 

 had somites consisting of four, six, and eight rings, respectively. To 

 those authors who lay much weight on the numljer of rings constitut- 

 ing a somite, from a systematic point of view, it might seem quite 

 strange, that species differing so much should be [)hiced in one and the 

 same genus. But as these leeches agree in almost every other character 

 than that of the number of the rings, I think it quite njitural to regard 

 them as members of a genus. 



Hind end. The hind end presents some peculiarities that are 

 common to all the three species. The anus opens on the dorsal surface 

 of the acetabulum, about a, ring's breadth from the posterior margin of 

 the last ring. This latter is distinct only on the dorsal side, but enters 

 into the formation of the acetabulum on the ventral side. Where the 

 anus opens is marked by a transverse furrow, and in Or. octonaria there 

 is still one more furrow behind the anus, so that we are apt to reckon 

 the dorsal surface of the acetalKilum as rings, if we count on that side. 

 Thus it is clear that in these species the acetal)ulum is not so distinctly 

 marked from the body as in (3ther genera. Fig. 14, PI. XXVIII, is a 

 sketch of the hind end of a very lîU'ge specimen of Or. octonaria, natural 

 size, and serves to show that the acetabulum is here, comparatively, very 

 small. The mode of life of these leeches seems to make the suctory 

 function of the acetabulum less necessary, and the growth of this 

 organ does not accompany that of the body, after the animal has 

 attained a certain dimension. 



Number of somites. By dissection we find that there are twenty- 



