BY E. J. GODDARD. 57 



Considering now the three species of Orohdella from this stand- 

 point, and examining the genital region, we find that the male 

 genital poi-e in the species is situated in i-eference to the sensory 

 annulus as follows : — 



0. Whitmani in 2nd annulus anterior to ganglion. 



0. Ijimai in 3rd 

 0. octonaria in 4th 



9> 



The nephridiopores are situated in 0. Whitmani and 0. Ijhnai 

 -on the posterior margin of the annulus preceding that in which 

 the ganglion lies. In the case of 0. octonaria the ganglia lie in 

 two annuli, and the nephridiopores on the posterior margin of 

 the annulus preceding the more anterior of these two annuli, so 

 that we may conclude that the same two annuli represent the 

 sensory annulus of 0. Whitmani and 0. Ijimai, and further that 

 the nephridiopores occupy the same relative position as in the 

 latter two species. Again, from the fact that the ganglion is so 

 situated in 0. octonaria, we may conclude from comparison with 

 the other two species that the sensory annulus remained quite 

 stable as regards somite-extension or increased annnlation in the 

 passage of the somite from the tetrannulate condition of 0. 

 Whitmani to that exemplified in 0. Ijimai with its somite com- 

 posed of six annuli. Later, however, in the passage to the con- 

 dition of the somite composed of eight annuli this sensory annulus 

 underwent division as shown in the manner above described in 

 O. octonaria. In tlie diagram shown in illustration of this I 

 have attempted to map out the order of origin of the annuli. 

 Somite-extension more readily affects the extreme annuli of the 

 somite, and we find that the anterior and posterior annuli of the 

 tetrannulate somite have divided to give rise to the somite of six 

 annuli. One may prove this by stating that, in the Hirudinea 

 in general, the terminal annuli are more prone to divide; that, 

 further, the sensor}' annulus, for reasons given above, appears 

 not to have been concerned in this stage; that the genital aper- 

 ture lies in the third annulus in front of the cansHon in 0. 

 Ijimai instead of being found in the second annulus reckoned in 



