xv SIPUNCULOIDEA HABITS 427 



nature of the sand and its method of locomotion may be readily 

 observed. A vigorous individual buries itself in a few moments 

 in the following manner : Running out the introvert to nearly 

 its full extent, and applying it to the surface of the sand till 

 some spot of less resistance is found, the animal still further 

 expands the introvert so that it penetrates the sand, provided 

 this is not too dense and firm, for then the body is merely 

 shoved backward. When the introvert is inserted, the con- 

 traction of the longitudinal muscles of the body-wall brings 

 the whole body forward somewhat, in case the introvert is fixed 

 in the sand. In case soft ooze was present, this fixation did 

 not take place, and the introvert was merely pulled out again, 

 but when the sand was of the right consistency the introvert 

 was fixed by becoming much swollen at the tip, and then con- 

 stricted just posterior to this swollen area. This bulb-like area 

 exerts lateral pressure on the sand, as could be seen by move- 

 ments of the grains. The swelling of the anterior end of the 

 introvert is brought about by the body-wall contracting else- 

 where, and forcing in liquid to distend that end. Owing to the 

 curved form assumed by the body in the normal contracted state 

 when first removed from its burrow, the entrance of the introvert 

 may often be nearly vertical, and hence the entire body is soon 

 raised nearly upright in the water above the sand. If the body 

 has thus been warped forward sufficiently to become somewhat 

 fixed in the sand, the introvert is rolled in and again thrust 

 forward from this new point of resistance, and so on till the 

 animal is entirely buried. This locomotion increases in speed 

 as the creature becomes more completely surrounded by sand, 

 and is the only means of moving from place to place. 



" On a smooth surface, or on one not presenting the right 

 degree of resistance, the Sipunculus does not change its position, 

 but remains till death finally occurs, rolling its introvert in and 

 out and contracting its body-wall to no purpose. 



" The essential factors in the mechanism bringing about this 

 hydrostatic locomotion are an elongated contractile sac filled with 

 liquid, and some means of definitely co-ordinating the contrac- 

 tions of the sac. 



" In natural environment the animals are found with some- 

 times one, sometimes the other end nearer the surface of the 

 sand : in the aquaria the same was observed, but when the 



