ELASTIC-RINGED ANIMALS. 149 
long stem there will follow it a splendid scarlet plume 
(/) arranged like a double fan, and waving in the 
water. The stopper with its stem is one of the 
tentacles of the worm enlarged at its end so as to 
shut the animal safely within the tube, while the 
other tentacles have become the beautiful plume 
which is the breathing apparatus of the animal. It is 
easy to understand that being in a tube, the Serpula, 
as this worm is called, cannot breathe through its 
skin like the leech or worm, and it needs these deli- 
cate gills to provide air for its body, while at the 
same time its sensitive nerves and apparatus of 
muscles enable it to draw them in like lightning when 
danger is near. 
There is an almost endless variety of these tube- 
building worms. You can scarcely pick up a piece 
of dark seaweed without finding upon it what look 
like very tiny shells (Sp, Fig. 5 3), but which are really 
coiled worm-tubes. Again you cannot search long 
among the sandy pools at low tide without finding 
some long tubes made of sand and broken pieces 
of shell wedged between the stones and rocks, and 
having forked sandy threads at their end. These 
tubes are the house of the Terebella or shell-binding 
worm, which selects particles of shell and sand with 
its tentacles and places them round its soft body, 
cementing them together as a mason cements the 
stones of a wall, till it forms a tube often a foot long, 
so firmly wedged into the beach that it is almost 
impossible to get one out perfect ; while you will 
rarely find the worm itself, as it draws back to the 
farthest end of the tube directly it is alarmed. 
These are the fixed seaworms, but there are 
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