380 PROCEEDINGS OF THE ACADEMY OF [May, 



arches and a remarkable system of imicellular valves. The ventral 

 vessel is free from the intestine, without sacculations and without 

 valves. It bifurcates in III, but is usually reunited in II by an asym- 

 metrical commissural vessel, from which a small azygous vessel arises. 

 The dorsal vessel is contractile, sacculated and provided with a set of 

 valves at each constriction throughout its entire length as far forward 

 as somite III. Although its anterior end is largely free from the intes- 

 tine, it retains a covering of chlorogogue cells as far as somite IV and 

 bifurcates only on reaching II. A pair of small lateral arches without 

 valves arises in I, and after dividing each joins the here double ventral 

 vessel. Beginning with III, the arches spring from the dorsal trunk at 

 the posterior ends of the somites just anterior to the valvular constric- 

 tions. They reach to the body walls, which they enter and then divide 

 into a complex integumental network emptying into the ventral 

 vessel by two trunks in each somite, while the plexuses of contiguous 

 somites are united by a longitudinal connecting vessel. The network 

 is similar to that of M. pilosus, but appears to be somewhat more open. 

 The lateral contractile trunks increase in size to X, one of which loops 

 into the sperm sac, while one in XI forms a rich plexus within the 

 ovisac. Behind XI they become much smaller, but continue to form 

 integumental plexuses which join the ventral vessel by two trunks. 

 The arches from III to XI are moniliform and provided with valves at 

 each constriction. That in III contains one set of valves, IV 2 sets, 

 V 2 or 3 sets, VI and VII 3 or 4 sets, and in VIII to XI 5 or 6 sets- 

 numbers which, while occasionally varying, are remarkably constant. 

 In the larger arches the two primary branches also may each contain 

 a set of valves. 



The brain (fig. 3) has a peculiarly lobulated surface and is broad and 

 very deeply incised anteriorly, somewhat narrower and scarcely emar- 

 ginated posteriorly, where the muscular sheath is produced into a pair 

 of contractile strands, between which the dorsal vessel passes. Ordi- 

 narily the brain is about i longer than broad, but the proportions vary 

 greatly with the state of contraction and extension of the worm. 



Nephridia (fig. 2) begin in VII with their nephrostomes opening into 

 VI ; none is present in X and XI, though a pair communicating with the 

 latter is frequently developed in XII. In young worms, in which the 

 transparent body wall permits the nephridia to be readily studied, they 

 are absent from many somites and developed on one side only of 

 others, but in full-grown worms few segments, except at the posterior 

 region, lack them. The funnel is small, with the prolonged lip or tongue 

 more slender and elongated than in M. pilosus. It passes into a short 



