ON MONOPYLEPHOEUS LIMOSUS. 



11 



setae are arranged in bundles in four longitudinal lines of the 

 body : two on the dorsal and two on the ventral side. The seta- 

 bundles begin in segment II and are absent on the ventral side of 

 segment XL In the anterior part of the body the setae lie almost 

 at the corners of the squarish cross-section of the body (fig. 7, 

 A), but behind the clitellum the bundles of the two sides gradually 

 approach each other toward the nerve cord, and as the posterior 

 segments are more or less flattened dorso-ventrally the setae occupy 

 the four corners of a trapezoid, as shown in figure 7, B. The 



d.V 



___„-■' d.s.b 



v.s.b. 



v.t n.c 



Fig. 7. 

 Semi-diagrammatic cross sections through, an anterior (A) and a 

 posterior (B) segment, showing the position of the seta-bundles, d.s.b. 

 — dorsal seta-bundle, d.v — dorsal vessel, al.c — alimentary canal, n.c — 

 ventral nerve cord, v.v — ventral vessel, v.s.b — ventral seta-bundle, n — 

 main mass of a nephridium. 



number of setae in a bundle is generally four in mature specimens, 

 with 1-3 young setae. To be more precise, it is three in segment 

 II as far as I have observed, and increasing in number posteriorly 

 becomes five or rarely six in the genital segments, but in the 

 greater middle part of the body it is mostly four ; more posteriorly 

 it decreases gradually and a seta-bundle consists there of only 2-1 

 complete setae. In the posteriormost segments the seta-bundles 

 are often formed of young setae only. Anteriorly to the clitellum 

 the seta -bundles are situated rather in the anterior region of a 



