THE ANNELIDA. 209 
their neighbourhood, also, branch out the respiratory organs. 
They are not, as is general in the annelids, in the form of fans, 
but arborescent, spreading out like fine branches. The tentacles 
appear at the first glance to be round threads; but upon a closer 
examination they are really found to be flat tubes, along whose 
surface run longitudinal grooves, sometimes so deep as to permit 
their sides to fold over, by this means enabling them to hold 
tightly to anything which they touch. In one species, the division 
between the channels is furnished with seriated teeth. The 
branchial apparatus is exceedingly beautiful, affording a great 
number of angles and curves and points; its colours are varied 
and brilliant. The protective tube of the terebella is formed as 
most of the others—of sand and fragments of shells; but the walls 
of its extremity are somewhat extended, to form a sheath for the 
tentacles and respiratory organs. If a terebella be placed in an 
aquarium devoid of its tube, it will extend its tentacles on every 
side, and actively commence to construct a new one. When a 
little of it has been made, the creature will creep into it, and lie 
perfectly still during the daylight. At evening it puts out its 
tentacles, and during the hours of the night works incessantly. 
How can an eyeless creature recognise the difference between 
day and night? 
In some of the species, the tentacles appear to divide the 
work: one confines itself to gathering material together, another 
transports it, a third places it in position and fixes it with mucus, 
whilst others gather up the dédris which falls from the work. 
The building continues for many hours uninterruptedly: the 
progress, if watched, appears but slow; however, next day, much 
has been done; for, during the night, the tower of the edifice has 
been lengthened—particles of sand have been regularly laid in 
order, and solidly united. With the dawn of the morning, the 
builder ceases, and rests from his labour, again to commence 
with renewed vigour when the night hides curious eyes from 
watching his proceedings. The interior of the tube is lined with 
a fine coating of silky matter, which binds together the masonry, 
and at the same time decorates the walls of the mansion. This 
material is produced from a humour secreted by the skin of the 
annelid—an excellent and economical liquid, which serves the 
