475 



placed far back on the large annulus, according to Oka at the line of 

 the furrow separating the large and small annuii, while in Microbdella 

 they are further forward, as in the Glossiphonidae generally , anterior 

 to the metamerie sensillae. In Microbdella the metamerie sensillae 

 are on the posterior half of the large ring. None have been described 

 for this Ozobranchus, but the branchia spring from the sides of the 

 corresponding region. The large and small annuii are clearly respec- 

 tively equivalent in the two forms. 



Just as Microbdella has been compared with Glossiphonia so may 

 the somite of Oka's Ozobranchus be compared with any triannulate 

 Ichthyobdellid, as, for example, Branchellion or Pontobdella. The 

 large ring of Pontobdella contains the nerve ganglion from which 



Microbdella Ozobranchus Pontobdella 





°^ 



YV 



V 



Diagrams of typical somites of Microbdella, Ozobranchus and Pontobdella. Upper 



series dorsal, lower ventral aspects, br stumps of branchia, gan ganglia, 



np nephropores. 



peripheral nerves, after passing through a small accessory ganglion, 

 diverge to the preceding and succeeding rings. Sometimes a fourth, 

 fifth or sixth narrow annulus is present as a Tesult of the subdivision 

 of one, two or all of the primary annuii, but for the present purposes 

 of comparison these conditions may be ignored. The large annulus 

 of Pontobdella also contains the principal papillae, the pulsating vas- 

 cular vesicles and the nephropores ; the latter vary in position but are 

 most frequently situated as shown in the figure , that is , well back on 

 the annulus. It seems extremely probable from these topographical 

 considerations that the large annulus of Ozobranchus corresponds to 

 the neural annulus plus its predecessor in Pontobdella and that its 

 structure fully confirms the results arrived at through a study of 

 Microbdella'. namely, that the neural annulus is morphologically the 



