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AUSTRALIAN HIRUDINEA, IV., 



The body is readily divisible into "neck-" and tmink-regions. 

 Along the trunk-region, in the mid-dorsal line, runs a longitudinal 

 groove, extending from the anterior extremity of that region 

 through the greater part of the body, 

 towards the posterior extremity. 



Annidi. — Total number visible on dorsal 

 surface is 55. Of these 16 fall in the 

 neck-region, and 39 in the trunk- region. 

 The annuli are very readily made out with 

 the naked eye, except at the anterior and 

 posterior extremities, where the lines 

 of division are not clearly marked. How- 

 ever, even in these regions, one can count 

 them definitely. 



Throughout the greater part of the 

 body the limits of the somites can be 

 seen, each consisting of three annuli, the 

 middle one of which is about double the 

 width of that anterior and posterior to 

 it. The surface of each of the smaller 

 annuli is divided dorsally into eight tuber- 

 cular areas, four on either side of the 

 mid-line; similarly on the ventral surface 

 where, however, the tubercles are not so 

 pronounced as on the upper surface. The 

 middle annulus of each somite is divided 

 dorsally into six tubercular areas; two 

 very large tubercles on either side of tlie 

 mid-line, between each of which two is a 

 minute tubercle; veutrally this annulus 

 is also divided into six areas, the two of 

 Fig.l— Diagram of Po7i- which lying next the mid-line are not 

 tohdella macrothela, shew- gQ pronounced as those lying to the outside 

 ing somitic constitution. , , 



of them. 



In the neck-region the tubercles are well developed (although 

 not so strongly as in the trunk-portion), so that the somites can 



