484 Illinois State Laboratory of Natural History. 



but become gradually less and less obvious as a 1 becomes 

 relatively larger and more completely dissociated from a 2. 

 At the same time all of the furrows and the emarginations 

 become more uniform ; but in some specimens of large size 

 and in a great many of the smaller ones the intersomitic 

 furrows remain more distinct and a l/«2 less distinct for the 

 entire length of the body, and especially is this so on the 

 ventral surface. The external limits of the somites are often 

 quite as distinctly indicated as in Microbdella, 



On the fully developed somites of the middle region a 3 is 

 always the largest annulus (PI. XLIL, Fig. 4), but, except in 

 the posterior somites of large individuals, there is no indica- 

 tion of further subdivision into b 5 and b 6. Anteriorly, and 

 especially in young individuals, a 2 is longer than a 1, but in 

 the middle region it is only equal to or slightly less than this. 

 Unlike P. rugosa this relative size of the annuli is the same 

 ventrally and dorsally, and the ventral furrows are either 

 exactly continuous with the dorsal or all are slightly and 

 equally in advance of them at the margins. 



At the posterior end, somite XXIII is triannulate above ; 

 but below the furrow a lja 2 becomes more or less reduced. 

 Dorsally the marginal curvatures and emarginations present 

 the features described for the anterior end, and a 3 is slightly 

 shorter than a 1 . 



XXIV. Although the annulus a 3 unites with a 2 mesially 

 and a 1 remains distinct for its entire width, contrary to the 

 relative disposition of these annuli anteriorly, the marginal 

 curvatures and emarginations still associate a 2 more closely 

 with its predecessor than with its successor. Marginally, at 

 least, a 1 is the largest component. On the ventral side all 

 three annuli unite more or less completely into one. 



XXV is very incomplete ; a 1 and a 2 are inseparable 

 and a 3 is distinct only marginally. XXVI is still more 

 faintly biannulate at the margins or even entirely unian- 

 nulate. XXVII is uniannulate. 



Reproductive Organs. — The six pairs of testes lie in the 

 posterior parts of somites XIII to XVIII, extending some- 

 what into the succeeding somite in each case. The vasa 



