130 



After an interval of a day or two in the mucus-coated tunnel the 

 young worm, for it is now an adult in miniature, has outgrown it and a new 

 tube is constructed. This is done by splitting the tube at a point where 

 the upright arm meets the horizontal portion, iu a U-shaped tube, or near 

 one end, in a straiglit tube, and then excavating a tunnel obliquely down- 

 wards and after nearly doubling the length of basal portion upwards to the 

 surface. This is its first lateral enlargement. The sand which the worm 

 excavates in constructing this extension is expelled from the opposite end 

 of ils first tube. The walls of the tunnel are coated with mucus as the 

 tunnel advances, so that the U-shaped tube is completed when the exca- 

 vation reaches the surface. The tube becomes strengthened from time to 

 time by additional layers of mucus that hardens to form a parchment-like 

 material that gives the older tubes a laminated structure. They are 

 enlarged in the same vertical plane unless prevented from doing so by 

 some obstruction, as a shell, when they turn obliquely along the surface 

 of the obstruction or abandon the new enlargement and construct an en- 

 largement from the opposite end of the tube. Two or three days later the 

 process is repeated, possibly by the extension of the opposite end of the tube. 

 The horizontal portion of each new enlargement is larger iu diameter and 

 is buried dee]ier in the sand than the tube from which it is a branch 

 (Fig. 4). Enlargements are frequently of such length as to double the size 

 of the U-tube, and are completed to the surface of the sand iu from twenty- 

 four to forty-eight hours. They are made indltferently at one end or other 

 of the smaller tube. The fate of the intermediate tubes has been discussed 

 in another part of the present paper. 



The burrov.-ing is done by the anterior region of the worm. Its setiger- 

 ous segments dislodge the sand and pass it to the middle and posterior 

 regions of the body, and they convey it backwards into the tube by the 

 combined contraction and expansion of the body, and the rythmic move- 

 ments of the palettes and lobes of the segments. The worm ceases burrow- 

 ing at intervals of a few minutes and expels the accumulated sand at one 

 end of the tube around which it falls and forms a mound ; the other end, 

 or intermediate tube, is the incnrrent tube so long as the burrowing is in 

 progress, but when the new burrow is complete a septum of parchment is 

 formed across the base of the ijitermediate tube and it ceases to be of any 

 use to the worm. 



The worms which form their tubes in aquaria with a thin layer of sand 



