ON HAPLOBSANCHUS. 171 



the blade is shorter and wider ; in both cases the blade only 

 occurs on one side of the axis. 



The third somite has similar setse but slightly more nume- 

 rous (the usual number of the various kinds of setae is accu- 

 rately shown in fig. 2), and bears in addition ventral "crochet" 

 setse (fig. 10). These are not actually forked but evidently 

 correspond to the " crochet " setae of allied worms. 



The remaining somites in the ''thorax" bear similar setae, 

 the crochet setae present slight variations. Figs. 10 and 11 

 represent the extreme conditions, the capillary setae in the 

 posterior segments of the thorax gradually approach the con- 

 dition shown in fig. 12, intermediate in character between figs. 

 8 and 9. 



In the "abdomen," an inversion in the position of the setae 

 occurs, the capillary setae becoming ventral, and the crochet 

 setae dorsal, at the same time their character is changed (fig. I4) ; 

 the capillary setae become longer, more flexible, are often 

 bayonet-shaped (not well shown in fig. 13), and the blade 

 occurs on both sides of the axis ; the " crochet " setae become 

 very numerous and are closely placed in a transverse row, they 

 are finely serrated on one side at the free extremity. 



Alimentary canal. — The alimentary canal is simple, 

 there is a constriction between the fifth and sixth somites, and 

 between the sixth and seventh ; in the sixth somite it is dilated, 

 from the seventh somite to the anus, in the terminal somite, 

 it gradually narrows, immediately in front of the anus it is 

 slightly dilated (fig. 4). The canal is ciliated at any rate in its 

 posterior half, the anterior portion presents a brown pigment 

 in the walls. 



Blood-vessels. — The complete distribution of the circula- 

 tory system is not easily determined. 



The blood is green. 



There is a dorsal vessel which bifurcates in the peristomial 

 and terminal segments, the two branches in each case turning 

 round and uniting to form a ventral vessel. The dorsal and 

 ventral vessels are connected in the tenth and eleventh somites 



