[Illustration: FIG. 5.--A, _Autolytus_ (after Mensch) with numerous
buds. B, Portion of a colony of _Syllis ramosa_ (from M'Intosh).
_b.z_, Budding zone; p, anterior region of the parent worm; 1-5,
buds.]

[Illustration: FIG. 6.--A, Side view of the larva of _Lopadorhynchus_
(from Kleinenberg), showing the developing trunk region. B, Side view
of the trochophore larva of _Eupomatus uncinatus_ (from Hatschek).

A, Anus.
E, Eye.
M, Mouth.
ap, Apical organ.
h, "Head Kidney."
i, Intestine.
me, Mesoblast.
ms, Larval muscle.
o, Otocyst.
pp, Parapodium.
pr, Praeoral ciliated ring, or prototroch.]

As is very frequently the case with marine forms, as compared with
their fresh-water and terrestrial allies, the Polychaeta differ from
the Oligochaeta and Hirudinea in possessing a free living larval form
which is hatched at an early stage in development. This larva is
termed the Trochosphere larva, and typically (as it is held) is an
egg-shaped larva with two bands of cilia, one preoral and one
postoral, with an apical nervous plate surmounted by a tuft of longer
cilia, and with a simple bent alimentary canal, with lateral mouth and
posterior anus, between which and the ectoderm is a spacious cavity
(blastocoel) traversed by muscular strands and often containing a
larval kidney. The segmentation is of the mesoblast to begin with,
and appears later behind the mouth, the part anterior to this becoming
the prostomium of the adult. The chief modifications of this form are
seen in the _Mitraria_ larva of _Ammochares_ with only the preoral
band, which is much folded and which has provisional and long setae;
the atrochous larva, where the covering of cilia is uniform and not
split into bands; and the polytrochous larva where there are several
bands surrounding the body. There are also other modifications.

[Illustration: FIG. 7.--_Nereis pelagica_, L. (After Oersted.)]

[Illustration: FIG. 8.--_Sabella vesiculosa_, Mont. (After Montagu.)]

[Illustration: FIG. 9. _Arenicola marina_, L.]

_Classification_.--The older arrangement of the Polychaeta into
Errantia or free living and Tubicola or tube-dwelling forms will
hardly fit the much increased knowledge of the group. W.B. Benham's
division into Phanerocephala in which the prostomium is plain, and
Crytocephala in which the prostomium is hidden by the peristomium
ado