204 bulletin: museum of comparative zoology. 



Head directly continuous with body, and there is likewise no 

 demarcation between esophageal and intestinal regions, the thin 

 lateral margins of body extending forward to posterior ends of 

 cephalic furrows. Head very broad, the mouth being situated 

 between posterior ends of cephalic furrows, and in preserved speci- 

 mens almost as near to tip of snout as to posterior end of either 

 cephalic furrow. Cephalic furrows remarkably short, measuring but 

 4 mm. in length in a preserved specimen whose M'idth is 27 mm, in 

 intestinal region ; they are thus about one seventh as long as the 

 greatest width of body (PI. 4, fig. 41). 



Esophageal region very sliort. much flattened, scarcely tAvice as 

 long as width of body, with very thin lateral margins continuous 

 with those of intestinal region. Even in esophageal region strong, 

 flattened bands of dorso-ventral muscles pass at regular intervals 

 beside proboscis sheath and esophagus from dorsal to ventral side of 

 body. 



Intestinal region very flat, with powerfully developed bands of 

 dorso-ventral muscles between the intestinal lobes. Internal longi- 

 tudinal muscles of body walls but very little developed. Lateral 

 margins of body extremely thin, extending laterally beyond the lat- 

 eral nerve cords on each side for a distance equal to nearly one 

 fifth the width of body, or much more than half as great as the 

 dorso-ventral thickness of body in same region. In a rather large 

 specimen the width of intestinal region after ])reservation measures 

 27 mm., while its thickness is but 7 mm. Intestinal region com- 

 monly remains concave on ventral surface, with a distinct longi- 

 tudinal ridge beneath each of the lateral nerves; from this point 

 out the lateral margins are extremely thin. 



Body tapers gradually towards posterior end, where the lateral 

 margins are even thinner than more anteriorly and are produced as 

 broad lateral fins (PI. 4, fig. 41). 



I know of no other species of the genus in which the worms are 

 so well adapted for swimming as in the species at hand, and experi- 

 ence with living individuals on the coast of Alaska has demon- 

 strated how actively and tirelessly they can propel themselves 

 through the water. It seems not unlikely that they often leave 

 their burrows to swim free in the water. 



Proboscis of remarkably large size, although inner longitudinal 

 muscular layer is practically wanting. Muscular crossings well 

 developed. 



