524 HoAvard Edwin Enders. 



of tLe obstruction or construct an enlargement from the opposite end 

 of the old tube. Two or three days later the process is repeated, pos- 

 sibly by an extension of the opposite end of the tube. The hori- 

 zontal portion of each new enlargement is larger in diameter and is 

 buried deeper in the sand than the tube from which it is a branch 

 (Fig. 2). The enlargements are frequently of such length as to 

 double the size of the U-tube, and are completed to the surface of the 

 sand in from twenty-four to forty-eight hours. They are made indif- 

 ferently 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 burrowing is done by the anterior region of the worm. Its 

 setigerous 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 tlie 

 rhythmic movements of the palettes and neuropodia. The worm 

 ceases burrowing at intervals of a few minutes and expels the accumu- 

 lated sand to the exterior in the same way that large quantities are 

 removed by individuals in glass tubes (p. 505). The sand that is 

 removed during the excavation is pushed out of one end of the tube 

 around which it falls and forms a conical mound; the other end, or 

 intermediate tube, is the incurrent tube so long as the burrowing is 

 in progress, but when the new burrow is complete a septum of parch- 

 ment is formed across the base of the intermediate 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 and diatoms on the bottom conform to the U-habit, though in 

 a horizontal plane, with one side of the tube cemented to the floor 

 of the vessel. 



The linear extensions are formed at such intervals as the rapid 

 growth of the worm requires. The length of the tubes, and the dates 

 on Avhich the enlargements were completed by two worms which I 

 reared from larvse taken in the tow-net, are as follows :^ 



='Both worms enlarged their tubes to 7G and 71 millimeters, respectively, 

 between September 12, when they were brought to the Biological Labora- 

 tory of the .Johns Hopkins University, and my return, October 4, 1905. The 



