MOVEMENTS OF THE ENTEROPNEUSTA. 257 



the whole animal is covered, the sheath being scarcely more than 

 one or two grains in thickness. The backward movement of the 

 sand on the collar and body is effected chiefly by the pushing 

 force of the ciliary action of the proboscis, though a weak ciliary 

 action on the collar and thorax also contributes to the result, as 

 does likewise muscular contractions of the proboscis. The pro- 

 lific secretion of mucus particularly by the proboscis and collar 

 serves to agglutinate the sand grains, to produce the sheath 

 above mentioned. This sheath forms a well-defined tube when 

 produced under the conditions here described so firm as to sup- 

 port itself in pieces of considerable size when pulled off and held 

 up in the water. 



Simultaneously with the ciliary action the animal is undergoing 

 almost constant contortions through muscular action ; but the 

 true nature and significance of the motions of this kind are not 

 seen until the creature is observed in the processes of burrowing. 

 Here the meaning of a characteristic movement of the proboscis 

 which is constantly noticed when the animal is not buried is 

 readily seen. I refer to the contraction waves, that move along 

 the organ from tip to base, but which often remain stationary in 

 the form of great blebs. 



When in its bnnwu these blebs act chiefly as hold-fasts by which, 

 through the contraction of the longitudinal muscles of the proboscis 

 and collar, the whole body is drawn forward. The muscles most 

 concerned in this are first those of the proboscis ; second, the 

 radio-longitudinal muscles of the collar ; and third, the longitu- 

 dinal muscles of the thorax-abdomen. 



The proboscis tip is driven forward in making a new burrow 

 partly by the action of the cilia, and partly by the contraction of 

 the layer of circular muscle fibers, the former apparently playing 

 the more important part! 



The wave motion so characteristic of the proboscis does not 

 appear on the collar, thorax, or abdomen, so that it plays no part 

 in the advancement of these portions of the body. The collar 

 shortens and elongates without great change in diameter, though 

 the anterior rim is sometimes narrowed down considerably, the 

 possible change in length of the part being nearly or quite one half 

 its entire length in the fully extended state. The shortening is un- 



