FIGURE 67. With a naked opercular peduncle, Serpula-stage.)

One of the simplest examples is furnished by the development of the
Tubicolar Annelids; but from its very simplicity it appears well adapted
to open the eyes of many who, perhaps, would rather not see, and it may
therefore find a place here. Three years ago I found on the walls of one
of my glasses some small worm-tubes (Figure 65), the inhabitants of
which bore three pairs of barbate branchial filaments, and had no
operculum. According to this we should have been obliged to refer them
to the genus Protula. A few days afterwards one of the branchial
filaments had become thickened at the extremity into a clavate operculum
(Figure 66), when the animals reminded me, by the barbate opercular
peduncle, of the genus Filograna, only that the latter possesses two
opercula. In three days more, during which a new pair of branchial
filaments had sprouted forth, the opercular peduncle had lost its
lateral filaments (Figure 67), and the worms had become Serpulae. Here
the supposition at once presents itself that the primitive tubicolar
worm was a Protula,--that some of its descendants, which had already
become developed into perfect Protulae, subsequently improved themselves
by the formation of an operculum which might protect their tubes from
inimical intruders,--and that subsequent descendants of these latter
finally lost the lateral filaments of the opercular peduncle, which
they, like their ancestors, had developed.

What say the schools to this case? Whence and for what purpose, if the
Serpulae were produced or created as ready-formed species, these lateral
filaments of the opercular peduncle? To allow them to sprout forth
merely for the sake of an invariable plan of structure, even when they
must be immediately retracted again as superfluous, would certainly be