1873.] aoe [Morse. 
barbed, and deciduous. In the young of all Brachiopods thus far 
observed, the sets are also very long, as above remarked. And Fritz 
Miiller has shown that in the 
Fig. 11. embryo of Discina there are Fig. 12. 
1 also remarkably barbed setz 
of great length, which are 
afterward discarded. Alex. \A 
Agassiz has also called atten- '\ \ 
| tion to the fact, that in Pal- yA 
\ eozoic worms the sets were Nh 
3 barbed. NA 
vi 
H) Tube Building. 
The fabricating of sand Discina. 
Nerine cirratulus. : 
tubes for the protection of Deciduous seta of 
larval Discina, from 
Fritz Miller. 
Deciduous seta of 
larval worm, Nerine the body is not a charac- 
ne aaa Cla teristic feature of Mollusks. 
Lima builds a peculiar “nest” 
in attaching pebbles and fragments of shells together by byssal 
threads, and imprisoning itself in that way. Gastrochcena also forms 
a flasked-shape cavity, in which it lives, and from which it has no 
means of withdrawing. These features in Mollusks may be said to 
bear only a remote resemblance to the tube building of worms. 
Jn worms, the building of tubes is a prominent feature of several 
sroups, from the lowest to the highest. 
Certain Rotifers, after attachment by their caudal portion, fabricate 
a sand tube into which they retract. (See Fig. 5.) 
Many sipunculoid worms occupy the dead shells of Dentalium, 
Littorina and other shells, and partially close the aperture, and even 
extend it by a mud tube of sonsiderable density. 
Of great importance, however, in these comparisons, is the fact 
that those worms, which are edentulous, which have the body 
divided into two regions—the thoracic and caudal, and which have 
a bi-lobed lophophore, the two arms often appearing spirally twisted, 
surrounding the mouth, and supporting ciliated cirri, are all 
famous tube builders. Sometimes the case is gelatinous or chitinous, 
often,the tube is calcareous, deposited in successive lines of growth, 
and resembling the shell of the Gasteropod ; but more frequently 
the tube is made of fine sand, mud, bits of shell, and coarser debris 
that the builder meets with. When Terebella is kept in confine- 
