56 AUSTRALIAN HIRUDINEA, V., 



is Still fused with it ventrally, the differentiation is not yet com- 

 plete. In somite v., we find there are three annuli, the middle 

 one of which is the sensory annulus; the posterior ring of the 

 somite is quite distinct dorsally and ventrally from the sensory 

 ring, but the anterior annulus is still fused ventrally with, or 

 rather not yet differentiated from, the sensory ring. Passing 

 now to somite vi., we Hnd the segment consisting still of three 

 annuli, all distinct from each other. In somite vii., there are four 

 annuli, the sensory ring being placed second; and in somite viii., 

 we find the first complete nuniVjer of annuli. 



From these facts it will l)e seen that we have a complete series 

 showing the development from the uniannulate somite of the 

 extremity of the bod}' to that which is pentannulate; and in L. 

 australis we get clearly the order of development of the annuli 

 in the somite, a fact which is most important in connection with 

 the condition of the complete somite. It will be seen, from the 

 facts stated above, that the sensory annulus gives rise by division 

 to another annulus posteriorly, later another anteriorly; then a 

 fourth develops at the posterior extremity of the somite, and 

 finally a fifth annulus at the anterior extremity. This is exactly 

 in keeping with the ideas of Whitman, Bristol, and Castle in 

 this direction. 



OrobdeUa. — This Japanese genus is represented by three species 

 of which 0. Whitmani is tetrannulate, 0. Jjimai has six, and 0. 

 octonaria has eight annuli entering into the formation of the 

 " unabbreviated " somite. We may reasonably regard 0. Whit- 

 mani as exhibiting the most primitive condition as regards the 

 constitution of the somite for the genus. As it has been shown, 

 in the case of Philcemon, Pontobdella, and Ozobranchus, that the 

 sensor)' annulus represents the third ring of the adult tetrannu- 

 late somite, we may, with every reason, conclude that the same 

 liolds in 0. Whitmani. This means that in this species the 

 ganglion of the ventral chain in the middle region of the Ijody is 

 found in the third annulus of the somite, and the nephridiupores 

 on tlie posterior margin of the second annulus. 



