82 E. A. ANDREWS. 



always forms an angle with the axis of the sac (Figs. 1-14). 

 To produce this abrupt change of angle and to form the spiral 

 tube the animal has the habit of modifying the anterior region 

 to make a sort of temporary head or cylindrical plug which 

 projects out of the sac and acts differently from the region of the 

 animal that remains within the sac. This plug-like head sticking 

 out of the sac like a cork in a bottle turns upward at a large 

 angle to the main axis of the sac, and thus generally at a large 

 angle with the surface of attachment, be it vertically above or 

 below. This angle in the main axis of the animal is maintained 

 during the making of the tube and the tube is made round about 

 the head region, little by little as the head region is extended 

 farther and farther away from the foot by the gradual elongation 

 of the whole animal. The length of the head or secreting region 

 is such as to produce one of the component rings of the tube and 

 it makes first the bottom one, then the next. However, these 

 are not commonly made as mere rings but as parts of a continuous 

 spiral, since, in proportion as the head slowly advances the head 

 revolves and bends laterally, or nutates, secreting successively 

 left, ventral, right, dorsal and so on. There are then three co- 

 ordinated actions in this making of a spiral tube: first, the slow 

 elongation of the animal, second the contractions of its anterior 

 end to make a mold or form about which secretion is deposited, 

 third the revolution of the head and also what might at first be 

 called a foutth, the localization of secretion so that it is not 

 uniform all round about at any one moment. This latter factor 

 is the least readily made out and involves the appreciation of the 

 fact that the spiral is really bounded by a hollow ridge due to a 

 duplication of the tube wall and this duplication arises from the 

 form and revolution. That is, as one side of the head is more 

 flat it makes the straight walls of tube but as opposite side is 

 bulged out it makes convex ridge and as revolution brings these 

 sides into play successively the bulged out secretion is lined 

 inside by the flat wall. The tube is smooth within but presents 

 a spiral hollow ridge on the outside only. 



The animal here considered proceeded normally with this 

 spiral activity till five or six rounds had been finished (Fig. 13) 

 and then made ready to complete the dwelling by the addition 



