pted by Sedgwick, can only be justified by the character used; for
the Terebellids, though phanerocephalous, have many of the features of
the Sabellids. It is perhaps safer to subdivide the Order into 6
Suborders (in the number of these following Benham, except in
combining the Sabelliformia and Hermelliformia). Of these 6, the two
first to be considered are very plainly separable and represent the
extremes of Polychaete organization, (1) _Nereidiformia_.--"Errant"
Polychaetes with well-marked prostomium possessing tentacles and palps
with evident and locomotor parapodia, supported (with few exceptions)
by strong spines, the aciculi; muscular pharynx usually armed with
jaws; septa and nephridia regularly metameric and similar throughout
body; free living and predaceous. (2) _Cryptocephala_.--Tube-dwelling
with body divided into thorax and abdomen marked by the setae, which
are reversed in position in the neuropodium and notopodium
respectively in the two regions. Parapodia hardly projecting; palps of
prosomium forming branched gills; no pharynx or eversible buccal
region; no septa in thorax, septa in abdomen regularly disposed.
Nephridia in two series; large, anterior nephridia followed by small,
short tubes in abdomen. The remaining groups are harder to define,
with the exception of the (3) _Capitelliformia_, which are mud-living
worms of an "oligochaetous" appearance, and with some affinities to
that order. The peristomium has no setae, and the setae generally are
hair-like or uncinate, often forming almost complete rings. The
genital ducts are limited to one segment (the 8th in _Capitella
capitata_), and there are genital setae on this and the next segment.
In other forms genital ducts and nephridia coexist in the same
segment. The nephridia are sometimes numerous in each segment. There
is no blood system, and the coelomic corpuscles contain haemoglobin.
(4) _Terebelliformia_. These worms are in some respects like the
Sabellids (Cryptocephala). The parapodia, as in the Capitellidae, are
hardly developed. The buccal region is unarmed and not eversible. The
prostomium has many long filaments which recall the gills of the
Sabellids, &c. The nephridia are specialized into two series, as in
the last-mentioned worms. (5) _Spioniformia_ (including
_Chaetopterus_, _Spio_, &c.) and (6) _Scoleciformia_ (_Arenicola_,
_Chloraema_, _Sternaspis_) are the remaining