133 



edges of a thicker laminated wall. This was true even in the smallest 

 specimens, No. 2 and No. 4, which I mentioned on page 131. The diameter 

 of their tubes was twice enlarged while they were thirty-eight and thirty- 

 two millimeters long, respectively, and before they constructed the next 

 linear enlargement. 



The outer surface of the tubes is everywhere coated with sand, except- 

 ing about the terminal portions that protrude above the sand flats in 

 which they are imbedded. Tliese terminal portions have one or more annu- 

 lations that give them the appearance of being formed of rings that dimin- 

 ish regularly in size upwards, so that the bases of the smaller rings are 

 overlapped by the top of the rings next below. Each ring represents 

 the successive height of the orifice, though not its diameter, for they are 

 split from time to time as I have just mentioned. They are moulded, 

 like the other ixtrtions of the tube, by the ventral lip of the buccal funnel, 

 and the length of each ring represents the height to which the lip was 

 extended when the ring was formed. The rings are, at first, very thin and 

 transparent but they become laminated by successive additions of mucus 

 to their inner walls. The laminae of which they are the free ends may 

 be separated with ease from those next below. 



SUMMARY. 



The principal points that I have attempted to bring out in this paper 

 are: 



1. The tubes are formed by the worm from mucus secreted by certain 

 cells of the body. Before the mucus hardens to a parchment-like material 

 it is molded by the A^entral lip of the buccal funnel. 



2. The tubes are first formed as tunnels in the diatoms, but later they 

 have the form of a U. 



3. The tubes are enlarged either in length or diameter or by a combi- 

 nation of both these methods. 



