iJV E. J. GODDAKD. 59* 



large anterior anuulus (sensor}'), and a small posterior annul us. 

 This constitution for a biannulate somite is l)y far the commoner 

 in all members of the Ilirudiuf". 



In some cases the somite of I'oufobdella consists of four annul i, 

 and then, says Castle, "Apparently, however, it is at the anterior 

 end, for in these animals which I have had an opportunity to- 

 examine, the new ring appears to he united more closely with 

 the ring which precedes than with that which follows a sensory 

 ring. Moreover the riug which piecedes the sensory ring is 

 usually not so broad as the one wliicii follows it. This is an 

 indication that it is the former rather than the latter wldch has 

 undergone division." I have not yet had the opportunity of 

 examining a species of Pontohdella in which thu souiite consists 

 of four annuli, but Castle's observations show very clearly that 

 in such forms the third aunulus is the sensory ring. 



Ozohranchihs hranchiatus. — Lately I have had the opportunity 

 of examinitig specimens of this form, and, inasmuch as the species 

 has not been noticed and examined in detail since Menzies 

 described it in 1791, the metamerism of such a species, since the 

 nature of the leech-body morphologically has been variously 

 interpreted since that time, should offer some little interest. 



In the " neck "-region of the young individual eight distinct 

 somites can lie made out, ami, judging from the intimacy of the 

 connection of the annuli, the limits of a somite are clearly marked 

 off. Eauh somite consists of an anterior annul u.s which is twice 

 the size of a posterior small annulus. In most instances, also, 

 but particularly in the posterior region of the " neck," there is a 

 more than faint indication of division in the large anterior 

 annulus, so that the somite is foreshadowed in its triannulate 

 condition. This fact shows that the large anterior annulus 

 represents within itself potentially the original primitive annulus 

 of the uniannulate somite; further, that the posterior annulus of 

 the somite is formed and definitel}^ diti'erentiated before traces of 

 the anterior annulus can be made out; and, again, the middle 

 annulus of the triannulate somite represents the primitive ring. 

 This is in keeping with what is to be observed in most cases in 



