60 .BULLETIN: MUSEUM OF COMPAEATIYE ZOOLOGY. 



The posterior section usually passes backward near the lateral face 

 of the sense organ, then bends medially to conform with the curv- 

 ature of the organ, and finally rises somewhat, bends a little forward, 

 and then ends 1)lindh'. 



The sensory epithelium lining the lateral face of this section of 

 the canal is very greatly modified, and consists of two lateral roAvs 

 of very large cells, often with more than a single nucleus each, and 

 each provided with a stiff process formed of a tuft of consolidated 

 cilia. Between the two rows of lateral cells are four rows of 

 smaller, so-called median cells all of which possess consolidated cilia 

 somewhat similar to those of the latei-al cells. All the cells of these 

 six rows are very highly specialized, and doubtless have an impor- 

 tant sensory function, the nature of which is not yet understood. 

 The other cells lining this section are similar to those of the anterior 

 portion of the canal. 



Two large masses of gland cells discharge their secretions into the 

 lumen of the canal. One of these chisters discharges at the point 

 separating the anterior fi'om tlie posterior section, as stated above, 

 while the other ])ours in its secretions some distance farther back. 



In the Iloplonemertea the sense organs are situated either in 

 front, beside, or beliind the brain. Those situated most anteriorly, 

 as in Emplectonema, are comparatively small, Avhile those lying 

 beside or behind the brain, as in Drepanophorus (PI. 24, fig. 181) 

 and some species of Amphiporus (PI. 21, fig. 129), are much larger. 

 There is actually much variation in size, i)Osition, and structure in 

 different species of the same genus. Of the 19 species of Amphi- 

 poRus from the Pacific coast in which the position of the sense 

 organ is known, 13 species have these organs situated in fi'ont of the 

 brain, while in the other 6 species they lie beside the brain, in the 

 angle between the dorsal and ventral ganglia, and in some of these 

 project some little distance posterior to brain. 



In the Iloplonemertea the anterior end of the ])osterior section of 

 the canal often divides into two distinct tubes, one of which swells 

 out into a sac-like sensory enlargement occu})ying the main ])ortion 

 of the organ, while the other continues behind the main portion as 

 a long, coiled glandular tube which ends blindly ])osteriorly. 



The sensory portions of the canal are surrounded with very 

 numerous nerve cells much like those of the dorsal ganglia with 

 which they are more or less continuous in the Ileteronemertea. In 



