36 The Microscope, 



of setse instead of the usual two. Each of the two parts of the com- 

 pound cluster is composed of from four to eight simple bristles of 

 unequal length, each entire compound fascicle being consequently 

 formed of from eight to sixteen setse, or of from two or four short 

 bristles intercalated between two or four long setre in each secondary 

 cluster. The long setae usually exceed in length the breadth of the 

 extended body, while the shorter ones are about one-half that length, 

 stouter, and gently curved at the distal extremities. None are fur- 

 cate. Each of the two parts composing each compound fascicle has 

 several (four or more) muscular threads which are independent of 

 those of the other secondary cluster, but both are connected by a 

 large and apparently strong muscle passing transversely from one 

 secondary bristle-sac to the other. As a rule there are four long and 

 four short setae in each of the separated parts. 



The oral segment is produced anteriorly as a large lip, which is 

 subcircular in outline, and soft and changeable in form. Its 

 entire lower surface is clothed with fine vibratile cilia. In u^. 

 venustum, Leidy states that the cilia fringe the edges of the hexa- 

 gonal cells composing the under surface of the lip, but such an 

 arrangement does not obtain with the present species, the whole sur- 

 face of the part being evenly clothed with the fibrillae. 



The anal segment is somewhat narrower than the other articula- 

 tions, obtusely rounded at the extremity, which is centrally emar- 

 ginate. Its posterior border, as well as the anterior margin of the 

 upper lip, is hispid with fine, short hairs. The general body surface 

 bears many similar short, scattered setae. 



The mouth is surrounded laterally and posteriorly by a thick, 

 muscular lip, shaped somewhat like the letter U, the arms extended 

 forwards. It is strongly ciliated and opens into a short but capa- 

 cious pharyngeal passage, which, at the second articulation, con- 

 tracts and is continued as a narrow, often tortuous, oesophagus, 

 which extends through the second segment to about the middle of 

 the third, where it dilates into the digestive cavity proper. The 

 latter is broadest centrally, and extends to near the sixth segment, 

 when it gradually narrows and is continued posteriorly to the termi- 

 nal anal aperture. The walls are thick, and the internal surface of 

 the entire alimentary canal is ciliated, the cilia of the rectum being 

 large and powerful, producing a strong current, and often rotating 

 the excrementitious mass before it is exj)elled. They frequently 

 project as a small cluster beyond the anal aperture. The cilia of the 

 remaining surfaces are fine and shoi-t. 



