360 W. S. NICKERSON. [Vol. XVII. 



When the American species is fixed by its foot to the bottom 

 of a glass vessel and bending its body in various directions, if 

 it be touched very gently upon one side by a needle, instead of 

 rotating out of harm's way, as L. annelidicola is described as 

 doing, it responds by drawing in its tentacles and contracting 

 its body. 



A band of circular fibers, the atrial sphincter, extends around 

 the margin of the lophophore in the velum, just over the bases 

 of the tentacles (PI. XXXII, Fig. 3). By its contraction the 

 infolding of the tentacles and the closure of the atrium are 

 accomplished. These fibers are shown as seen in section in 

 PI. XXXII, Figs. 5 and 7. 



Delicate fibers extend around the upper portion of the 

 oesophagus. By their contraction they reduce the lumen of 

 the organ (PI. XXXII, Fig. 2). 



Similar fine fibers surround the intestine and rectum, serving 

 as constrictors for these portions of the tube. 



A number of muscle fibers extend from either side of the 

 oesophagus backward and slightly upward over the stomach to 

 the vicinity of the body-wall on either side of the intestine. 

 Whether they attach to the sides of the oesophagus and intestine 

 respectively or to the body-wall near them, I have not determined, 

 but in either case they form virtually a band enclosing the two 

 structures and serving by their contraction to draw the intestine 

 forward and so to aid in the closure of the atrium. 



Muscle fibers also pass from the vicinity of the stomach ob- 

 liquely outward to the region on either side of the oesophagus 

 where the buds are formed, and in some cases may be seen 

 extending into the developing buds, to which they furnish a 

 mechanical support. On either side of the oviduct a few fibers 

 are found extending from the vicinity of the intestine upward 

 and forward into the mammary organ. A certain amount of 

 variability in the number and arrangement of the muscle fibers 

 traversing the part of the body between the stomach and the 

 floor of the atrium may reasonably be expected to occur, 

 dependent upon the presence of buds and the state of develop- 

 ment of the mammary organ. I have, however, no data for 

 determining the amount of such variability. 



