133 



cdKes of a thicker laminatt'd wall. This was true oven in the smallest 

 siK'ciiiit'iis, No. 2 and No. 4, wlii<-h I nieiitioucd on page 131. The diameter 

 of their tiii)es was twice enlarged while they were thirty -eight and thirty- 

 two millimeters long, respectively, and l)eforc they constructed the next 

 liiicnr 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. These terminal portions have one or more annu- 

 hitions 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 portions of the tube, by the veutral 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 sej)arated with ease from those next below. 



SUMMARY. 



The principal points that I liaA'e attempted to bring out in this paper 

 are: 



1. The tubes are formed by the worm from mucu.'< secreted by certain 

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

 it is molded by the ventral 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 lengtli or diameter or by a combi- 

 nation of both these methods. 



