BY K. J. GODDAKD. 6T 



specimens which may be deemed as mature, irrespective of details 

 as regards age. From this we conclude that, in Hcementeria, the 

 somite has not yet reached the pentannulate condition, and 

 further that this state would be reached by a division, evidently 

 simultaneous, of each of the terminal or non-sensory annuli of the 

 triannulate somite. The order of development of the annuli 

 would then be : — - 



12 3 4 5 



d c a b d 



This order shows then again a variation from that which obtains 

 in such pentannulate forms as Hirudo, ami is intermediate 

 between that which obtains in the latter forms and such forms 

 as Ozohranchus, PontobdelLa, Orohdella, and Philcenion. A point 

 of further interest in connection with this is that the triannulate 

 adult .somite is the prevalent condition among the Rhynchohdellidce, 

 and tlie pentannulate among the Arhy nchobdellidce. 



Hcementeria is the only member of the lihyachobellidce which 

 shows any trace of the pentannulate somite, and occupying an 

 equally UTiique position among the ArlcynchobdelUdce we find 

 Mesobdella which is th« only member of the latter group in which 

 the adult somite is triannulate. Both these genera then serve as 

 intermediate forms between the Iihy)ichobdellidce and Arhynchoh- 

 dellidce in connection with the nature of the somite. 



Summari/.—Fvom facts and .statements given above, it may be 

 concluded in general that the posterior region of the sensory 

 somite is first aflfected in connection with somite-extension. 

 Furtiier divisions may affect the posterior region exclusively as 

 in Semilageneta and Geobdella, but in the majority of cases we find 

 the anterior similarly aflPected. The fact that the division is 

 proved to be restricted to the posterior region iu some forms is 

 not astounding, (and is very interesting in keeping with the fact 

 that the posterior annulus of the l)iannulate somite is the first 

 non-sen.sory annidus developed in that it shows a distinct stimulus 

 behind this region) in regard to division, and inasmuch as con- 

 siderable variation takes place in the evolution of the multianu- 

 late condition of the somite is but to ba expected. In the 



