Chapter VI: Part 6

See a volume on _The Evils of Disunity in Central and Local
Administration ... and the New Centralization for the People_, by
Edwin Chadwick (1885); also _The Health of Nations, a Review of the
Works of Edwin Chadwick, with a Biographical Introduction_, by Sir B.
W. Richardson (1887).

CHAEREMON, Athenian dramatist of the first half of the 4th century B.C. He is generally considered a tragic poet. Aristotle (_Rhetoric_, iii. 12) says his works were intended for reading, not for representation. According to Suidas, he was also a comic poet, and the title of at least one of his plays (_Achilles Slayer of Thersites_) seems to indicate that it was a satyric drama. His _Centaurus_ is described by Aristotle (_Poet._ i. 12) as a rhapsody in all kinds of metres. The fragments of Chaeremon are distinguished by correctness of form and facility of rhythm, but marred by a florid and affected style reminiscent of Agathon. He especially excelled in descriptions (irrelevantly introduced) dealing with such subjects as flowers and female beauty. It is not agreed whether he is the author of three epigrams in the Greek Anthology (Palatine vii. 469, 720, 721) which bear his name.

See H. Bartsch, _De Chaeremone Poeta tragico_ (1843); fragments in A.
Nauck, _Fragmenta Tragicorum Graecorum_.

CHAEREMON, of Alexandria (1st century A.D.), Stoic philosopher and grammarian. He was superintendent of the portion of the Alexandrian library that was kept in the temple of Serapis, and as custodian and expounder of the sacred books ([Greek: ierogrammateus] sacred scribe) belonged to the higher ranks of the priesthood. In A.D. 49 he was summoned to Rome, with Alexander of Aegae, to become tutor to the youthful Nero. He was the author of a _History of Egypt_; of works on _Comets, Egyptian Astrology_, and _Hieroglyphics_; and of a grammatical treatise on _Expletive Conjunctions_ ([Greek: syndesmoi parapleropaeromatikoi]). Chaeremon was the chief of the party which explained the Egyptian religious system as a mere allegory of the worship of nature. His books were not intended to represent the ideas of his Egyptian contemporaries; their chief object was to give a description of the sanctity and symbolical secrets of ancient Egypt. He can hardly be identical with the Chaeremon who accompanied (c. 26 B.C.; Strabo xvii. p. 806) Aelius Gallus, praefect of Egypt, on a journey into the interior of the country.

Fragments in C. Muller, _Fragmenta Historicorum Graecorum_, iii.
495-499.

CHAERONEIA, or CHAERONEA, an ancient town of Boeotia, said by some to be the Homeric Arne, situated about 7 m. W. of Orchomenus. Until the 4th century B.C. it was a dependency of Orchomenus, and at all times it played but a subordinate part in Boeotian politics. Its importance lay in its strategic position near the head of the defile which presents the last serious obstacle to an invader in central Greece. Two great battles were fought on this site in antiquity. In 338 B.C. Philip II. and Alexander of Macedon were confronted by a confederate host from central Greece and Peloponnese under the leadership of Thebes and Athens, which here made the last stand on behalf of Greek liberty. A hard-fought conflict, in which the Greek infantry displayed admirable firmness, was decided in favour of Philip through the superior organization of his army. In 86 B.C. the Roman general L. Cornelius Sulla defeated the army of Mithradates VI., king of Pontus, near Chaeroneia. The latter's enormous numerical superiority was neutralized by Sulla's judicious choice of ground and the steadiness of his legionaries; the Asiatics after the failure of their attack were worn down and almost annihilated. Chaeroneia is also notable as the birthplace of Plutarch, who returned to his native town in old age, and was held in honour by its citizens for many successive generations. Pausanias (ix. 40) mentions the divine honours accorded at Chaeroneia to the sceptre of Agamemnon, the work of Hephaestus (cf. _Iliad_, ii. 101). The site of the town is partly occupied by the village of Kapraena; the ancient citadel was known as the Petrachus, and there is a theatre cut in the rock. A colossal seated lion a little to the S.E. of the site marks the grave of the Boeotians who fell fighting against Philip; this lion was found broken to pieces; the tradition that it was blown up by Odysseus Androutsos is incorrect (see Murray, _Handbook for Greece_, ed. 5, 1884, p. 409). It has now been restored and re-erected (1905).

AUTHORITIES.--Thucydides iv. 76; Diodorus xvi. 85-86; Plutarch,
_Alexander_, ch. 9; _Sulla_, chs. 16-19; Appian, _Mithradatica_, chs.
42-45; W.M. Leake, _Travels in Northern Greece_ (London, 1835), ii.
112-117, 192-201; B.V. Head, _Historia Numorum_ (Oxford, 1887), p.
292; J. Kromayer, _Antike Schlachtfelder in Griechenland_ (Berlin,
1903), pp. 127-195; G. Sotiriades in _Athen. Mitteil._ 1903, pp. 301
ff.; 1905, p. 120; 1906, p. 396; [Greek: Ephem. Archaiol.], 1908, p.
65.

CHAETOGNATHA, the name given by R. Leuckhart to a small group of transparent and for the most part pelagic organisms, whose position in the animal kingdom is a very isolated one. Only three genera, _Sagitta_, _Spadella_ and _Krohnia_, are recognised, and the number of species is small. Nevertheless these animals exist in extraordinary quantities, so that at certain seasons and under certain conditions the surface of the sea seems almost stiff with the incredible multitude of organisms which pervade it. Rough seas, &c., cause them to seek safety in dropping into deeper water. Deep-sea forms also occur, but in spite of this the group is essentially pelagic.

St, Septa dividing body-cavity transversely.
g^2, Cerebral ganglia.
n^1, Commissure uniting this with ventral ganglion (not shown in fig.).
n^2, Nerve uniting cerebral ganglia with small ganglia on head.
nr, Olfactory nerve.
d, Alimentary canal.
r, Olfactory organ.
te, Tentacle.
t, Tactile hairs springing from surface of body.
e, Ovary.
el, Oviduct.
ho, Testes.
sg, Vas deferens.
f^2, f^3, Lateral and caudal fins.
sb, Seminal pouch.

The eyes are indicated as black dots behind the cerebral ganglia.]

As a rule the body is some 1 to 2 or 3 cm. in length, though some
species are larger, by 4 or 5 mm. in breadth, and it is shaped
something like a torpedo with side flanges and a slightly swollen,
rounded head. It can be divided into three regions--(i.) head, (ii.)
trunk, and (iii.) tail, separated from one another by two transverse
septa. The almost spherical head is covered by a hood which can be
retracted; it bears upon its side a number of sickle-shaped, chitinous
hooks and one or more short rows of low spines--both of these features
are used in characterizing the various species. A pair of eyes lie
dorsally and behind them is a closed circlet, often pulled out into
various shapes, of modified epidermis, to which an olfactory function
has been attributed. The interior of the head is filled up with masses
of muscle fibres which are mainly occupied with moving the
sickle-shaped hooks. The trunk contains a spacious body-cavity filled
during the breeding season by the swollen ovaries, and the same is
true of the tail if we substitute testes for ovaries.

The skin consists of a transparent cuticle excreted by the underlying
ectoderm, the cells of which though usually one-layered may be heaped
up into several layers in the head; beneath this is a basement
membrane, and then a layer of longitudinal muscle fibres which are
limited inside by a layer of peritoneal cells. The muscles are
striated and arranged in four quadrants, two dorso-lateral and two
ventro-lateral, an arrangement which recalls that of the Nematoda,
whilst in their histology they somewhat resemble the muscles of the
Oligochaeta. Along each side of the body stretches a horizontal fin
and a similar flange surrounds the tail. Into these fins, which are
largely cuticular and strengthened by radiating bars, a single layer
of ectoderm cells projects.

The mouth, a longitudinal slit, opens on to the ventral surface of the
head. It leads into a straight alimentary canal whose walls consist of
a layer of ciliated cells ensheathed in a thin layer of peritoneal
cells. There is no armature, and no glands, and the whole tract can
only be divided into an oesophagus and an intestine. The latter runs
with no twists or coils straight to the anus, which is situated at the
junction of the trunk with the tail. A median mesentery running
dorso-ventrally supports the alimentary canal and is continued behind
it into the tail, thus dividing the body cavity into two lateral
halves.

There are no specialized circulatory, respiratory or excretory organs.

The nervous system consists of a cerebral ganglion in the head, a
conspicuous ventral ganglion in the trunk, and of lateral commissures
uniting these ganglia on each side. The whole of this system has
retained its primitive connexion with the ectoderm. The cerebral
ganglion also gives off a nerve on each side to a pair of
small-ganglia, united by a median commissure, which have sunk into and
control the muscles of the head. As in other animals there is a minute
but extensive nervous plexus, which permeates the whole body and takes
its origin from the chief ganglia. In addition to the eyes and the
olfactory circle on the head scattered tactile papillae are found on
the ectoderm.

Chaetognatha are hermaphrodite. The ovaries are attached to the side
walls of the trunk region; between them and the body wall lie the two
oviducts whose inner and anterior end is described as closed, their
outer ends opening one on each side of the anus, where the trunk joins
the tail. According to Miss N.M. Stevens the so-called oviduct acts
only as a "sperm-duct" or receptaculum seminis. The spermatozoa enter
it and pass through its walls and traverse a minute duct formed of two
accessory cells, and finally enter the ripe ovum. Temporary oviducts
are formed between the "sperm-duct" and the germinal epithelium at
each oviposition. A number of ova ripen simultaneously. The two testes
lie in the tail and are formed by lateral proliferations of the living
peritoneal cells. These break off and, lying in the coelomic fluid,
break up into spermatozoa. They pass out through short vasa deferentia
with internal ciliated funnels, sometimes an enlargement on their
course--the seminal vesicles--and a minute external pore situated on
the side of the tail.

With hardly an exception the transparent eggs are laid into the sea
and float on its surface. The development is direct and there is no
larval stage. The segmentation is complete; one side of the hollow
blastosphere invaginates and forms a gastrula. The blastopore closes,
a new mouth and a new anus subsequently arising. The archenteron gives
off two lateral pounchs and thus becomes trilobed. The middle lobe
forms the alimentary canal; it closes behind and opens to the exterior
anteriorly and so makes the mouth. The two lateral lobes contain the
coelom; each separates off in front a segment which forms the head and
presumably then divides again to form anteriorly the trunk, and
posteriorly the tail regions. An interesting feature of the
development of Chaetognaths is that, as in some insects, the cells
destined to form the reproductive organs are differentiated at a very
early period, being apparent even in the gastrula stage.

The great bulk of the group is pelagic, as the transparent nature of
all their tissues indicates. They move by flexing their bodies.
_Spadella cephaloptera_ is, however, littoral and oviposits on
seaweed, and the "Valdivia" brought home a deep-sea species.

The three genera are differentiated as follows:--

_Sagitta_ M. Slabber, with two pairs of lateral fins. This genus was
named as long ago as 1775.

_Krohnia_ P. Langerhans, with one lateral fin on each side, extending
on to the tail.

_Spadella_ P. Langerhans, with a pair of lateral fins on the tail and
a thickened ectodermic ridge running back on each side from the head
to the anterior end of the fin.

The group is an isolated one and should probably be regarded as a
separate phylum. It has certain histological resemblances with the
Nematoda and certain primitive Annelids, but little stress must be
laid on these. The most that can be said is that the Chaetognaths
begin life with three segments, a feature they share with such
widely-differing groups as the Brachiopoda, the Echinoderma and the
Enteropneusta, and probably Vertebrata generally.

See O. Hertwig, _Die Chaetognathen, eine Monographie_ (Jena, 1880);
B.J. Grassi, _Chetognathi: Flora u. Fauna d. Golfes von Neapel_
(1883); S. Strodtman, _Arch. Naturg._ lviii., 1892; N.M. Stevens,
_Zool. Jahrb. Anat._ xviii., 1903, and xxi., 1905. (A. E. S.)

CHAETOPODA (Gr. [Greek: chaite], hair, [Greek: pous], foot), a zoological class, including the majority of the Annelida (q.v.), and indeed, save for the Echiuroidea (q.v.), co-extensive with that group as usually accepted. They are divisible into the Haplodrili (q.v.) or Archiannelida, the Polychaeta containing the marine worms, the Oligochaeta or terrestrial and fresh-water annelids (see EARTHWORM), the Hirudinea or leeches (see LEECH), and a small group of parasitic worms, the Myzostomida (q.v.).

The distinctive characters of the class Chaetopoda as a whole are partly embodied in the name. They possess (save for certain Archiannelida, most Hirudinea, and other very rare exceptions) setae or chaetae implanted in epidermal pits. The setae are implanted metamerically in accordance with the metamerism of the body, which consists of a prostomium followed by a number of segments. The number of segments in an individual is frequently more or less definite. The anterior end of body always shows some "cephalization." The internal organs are largely repeated metamerically, in correspondence with the external metamerism. Thus the body cavity is divided into a sequence of chambers by transverse septa; and even among the Hirudinea, where this condition is usually not to be observed, there is embryological evidence that the existing state of affairs is derived from this. Commonly the nephridia are strictly paired a single pair to each segment, while the branches of the blood vascular system are similarly metameric. The alimentary canal is nearly always a straight tube running from the mouth, which is surrounded by the first segment of the body and overhung by the prostomium, to the anus, which is then either surrounded by the last segment of the body or opens dorsally a little way in front of this.

THE CLASS AS A WHOLE.--The Chaetopoda are with but few exceptions (Myzostomida in part, _Sternaspis_) elongated worms, flattened or, more usually, cylindrical, and bilaterally symmetrical. The body consists of a number of exactly similar or closely similar segments, which are never fused and metamorphosed, as in the Arthropoda, to form specialized regions of the body. It is, however, always possible to recognize a head, which consists at least of the peristomial segment with a forward projection of the same, the prostomium. A thorax also is sometimes to be distinguished from an abdomen. Where locomotive appendages (the parapodia of the Polychaeta) exist, they are never jointed, as always in the Arthropoda; nor are they modified anteriorly to form jaws, as in that group.

E, Eye.
M, Mouth.
d.c, Dorcal cirrus.
per, Peristomium, probably equal to two segments,
per.c, Peristomial cirri.
pl, Prostomial palp.
pp, Parapodium.
pr, Prostomium.
pr.t, Prostomial tentacle.
t.s, Trunk segment.
v.c, Ventral cirrus.]

The prostomium overhangs the mouth, and is often of considerable size
and, as a rule, quite distinct from the segment following, being
separated by an external groove, and containing, at least temporarily,
the brain, which always arises there. Its cavity also is at first
independent of the coelom though later invaded by the latter. In any
case the cavity of the prostomium is single, and not formed, as is the
cavity of the segments of the body, by paired coelomic chambers. It
has, however, been alleged that this cavity is formed by a pair of
mesoblastic somites (N. Kleinenberg), in which case there is more
reason for favouring the view that would assign an equality between
the prostomium and the (in that case) other segments of the body. The
peculiar prostomium of _Tomopteris_ is described below. The body wall
of the Chaetopoda consists of a "dermo-muscular" tube which is
separated from the gut by the coelom and its peritoneal walls, except
in most leeches. A single layer of epidermic cells, some of which are
glandular, forms the outer layer. Rarely are these ciliated, and then
only in limited tracts. They secrete a cuticle which never approaches
in thickness the often calcified cuticle of Arthropods. Below this is
a circular, and below that again a longitudinal, layer of muscle
fibres. These muscles are not striated, as they are in the Arthropoda.

_Setae_.--These chitinous, rod-like, rarely squat and then hook-like
structures are found in the majority of the Chaetopoda, being absent
only in certain Archiannelida, most leeches, and a very few
Oligochaeta. They exist in the Brachiopoda (which are probably not
unrelated to the Chaetopoda), but otherwise are absolutely distinctive
of the Chaetopods. The setae are invariably formed each within an
epidermic cell, and they are sheathed in involutions of the epidermis.
Their shape and size varies greatly and is often of use in
classification. The setae are organs of locomotion, though their large
size and occasionally jagged edges in some of the Polychaeta suggest
an aggressive function. They are disposed in two groups on either
side, corresponding in the Polychaeta to the parapodia; the two
bundles are commonly reduced among the earthworms to two pairs of
setae or even to a single seta. On the other hand, in certain
Polychaeta the bundles of setae are so extensive that they nearly form
a complete circle surrounding the body; and in the Oligochaet genus
_Perichaeta_ (= _Pheretima_), and some allies, there is actually a
complete circle of setae in each segment broken only by minute gaps,
one dorsal, the other ventral.

_Coelom_.--The Chaetopoda are characterized by a spacious coelom,
which is divided into a series of chambers in accordance with the
general metamerism of the body. This is the typical arrangement, which
is exhibited in the majority of the Polychaeta and Oligochaeta; in
these the successive chambers of the coelom are separated by the
intersegmental septa, sheets of muscle fibres extending from the body
wall to the gut and thus forming partitions across the body. The
successive cavities are not, however, completely closed from each
other; there is some communication between adjoining segments, and the
septa are sometimes deficient here and there. Thus in the Chaetopoda
the perivisceral cavity is coelomic; in this respect the group
contrasts with the Arthropoda and Molluscs, where the perivisceral
cavity is, mainly at least, part of the vascular or haemal system, and
agrees with the Vertebrata. The coelom is lined throughout by cells,
which upon the intestine become large and loaded with excretory
granules, and are known as chloragogen cells. Several forms of cells
float freely in the fluid of the coelom. In another sense also the
coelom is not a closed cavity, for it communicates in several ways
with the external medium. Thus, among the Oligochaeta there are often
a series of dorsal pores, or a single head pore, present also among
the Polychaeta (in _Ammochares_). In these and other Chaetopods the
coelom is also put into indirect relations with the outside world by
the nephridia and by the gonad ducts. In these features, and in the
fact that the gonads are local proliferations of the coelomic
epithelium, which have undergone no further changes in the simpler
forms, the coelom of this group shows in a particularly clear fashion
the general characters of the coelom in the higher Metazoa. It has
been indeed largely upon the conditions characterizing the Chaetopoda
that the conception of the coelom in the Coelomocoela has been based.

Among the simpler Chaetopoda the coelom retains the character of a
series of paired chambers, showing the above relations to the exterior
and to the gonads. There are, however, further complications in some
forms. Especially are these to be seen in the more modified
Oligochaeta and in the much more modified Hirudinea. In the
Polychaeta, which are to be regarded as structurally simpler forms
than the two groups just referred to, there is but little subdivision
of the coelom of the segments, indeed a tendency in the reverse
direction, owing to the suppression of septa. Among the Oligochaeta
the dorsal vessel in _Dinodrilus_ and _Megascolides_ is enclosed in a
separate coelomic chamber which may or may not communicate with the
main coelomic cavity. To this pericardial coelom is frequently added a
gonocoel enclosing the gonads and the funnels of their ducts. This
condition is more fully dealt with below in the description of the
Oligochaeta. The division and, indeed, partial suppression of the
coelom culminates in the leeches, which in this, as in some other
respects, are the most modified of Annelids.

_Nervous System._--In all Chaetopods this system consists of cerebral
ganglia connected by a circumoesophageal commissure with a ventral
ganglionated cord. The plan of the central nervous system is therefore
that of the Arthropoda. Among the Archiannelida, in _Aeolosoma_ and
some Polychaetes, the whole central nervous system remains imbedded in
the epidermis. In others, it lies in the coelom, often surrounded by a
special and occasionally rather thick sheath. The cerebral ganglia
constitute an archicerebrum for the most part, there being no evidence
that, as in the Arthropoda, a movement forward of post-oral ganglia
has taken place. In the leeches, however, there seems to be the
commencement of the formation of a syncerebrum. In the latter, the
segmentally arranged ganglia are more sharply marked off from the
connectives than in other Chaetopods, where nerve cells exist along
the whole ventral chain, though more numerous in segmentally disposed
swellings.

_Vascular System._--In addition to the coelom, another system of
fluid-holding spaces lies between the body wall and the gut in the
Chaetopoda. This is the vascular or haemal system (formerly and
unnecessarily termed pseudhaemal). With a few exceptions among the
Polychaeta the vascular system is always present among the Chaetopoda,
and always consists of a system of vessels with definite walls, which
rarely communicate with the coelom. It is in fact typically a closed
system. The larger trunks open into each other either directly by
cross branches, or a capillary system is formed. There are no lacunar
blood spaces with ill-defined or absent walls except for a sinus
surrounding the intestine, which is at least frequently present. The
principal trunks consist of a dorsal vessel lying above the gut, and a
ventral vessel below the gut but above the nervous cord. These two
vessels in the Oligochaeta are united in the anterior region of the
body by a smaller or greater number of branches which surround the
oesophagus and are, some of them at least, contractile and in that
case wider than the rest. The dorsal vessel also communicates with the
ventral vessel indirectly by the intestinal sinus, which gives off
branches to both the longitudinal trunks, and by tegementary vessels
and capillaries which supply the skin and the nephridia. In the
smaller and simpler forms the capillary networks are much reduced, but
the dorsal and ventral vessels are usually present. The former,
however, is frequently developed only in the anterior region of the
body where it emerges from the peri-intestinal blood sinus. On the
other hand, additional longitudinal trunks are sometimes developed,
the chief one of which is a supra-intestinal vessel lying below the
dorsal vessel and closely adherent to the walls of the oesophagus in
which region it appears. The capillaries sometimes (in many leeches
and Oligochaeta) extend into the epidermis itself. Usually they do not
extend outwards of the muscular layers of the body wall. The main
trunks of the vascular system often possess valves at the origin of
branches which regulate the direction of the blood flow. Among many
Oligochaeta the dorsal blood-vessel is partly or entirely a double
tube, which is a retention of a character shown by F. Vezhdovsky to
exist in the embryo of certain forms. The blood in the Chaetopoda
consists of a plasma in which float a few corpuscles. The plasma is
coloured red by haemoglobin: it is sometimes (in _Sabella_ and a few
other Polychaeta) green, which tint is due to another respiratory
pigment. The plasma may be pink (_Magelona_) or yellow (_Aphrodite_)
in which cases the colour is owing to another pigment. In _Aeolosoma_
it is usually colourless. The vascular system is in the majority of
Chaetopods a closed system. It has been asserted (and denied) that the
cellular rod which is known as the "Heart-body" (_Herzkorper_), and is
to be found in the dorsal vessel of many Oligochaeta and Polychaeta,
is formed of cells which are continuous with the chloragogen cells,
thus implying the existence of apertures of communication with the
coelom. The statement has been often made and denied, but it now seems
to have been placed on a firm basis (E.S. Goodrich), that among the
Hirudinea the coelom, which is largely broken up into narrow tubes,
may be confluent with the tubes of the vascular system. This state of
affairs has no antecedent improbability about it, since in the
Vertebrata the coelom is unquestionably confluent with the haemal
system through the lymphatic vessels. Finally, there are certain
Polychaeta, _e g._ the _Capitellidae_, in which the vascular system
has vanished altogether, leaving a coelom containing
haemoglobin-impregnated corpuscles. It has been suggested (E. Ray
Lankester) that this condition has been arrived at through some such
intermediate stage as that offered by Polychaet _Magelona_. In this
worm the ventral blood-vessel is so swollen as to occupy nearly the
whole of the available coelom. Carry the process but a little farther
and the coelom disappears and its place is taken by a blood space or
haemocoel. It has been held that the condition shown in certain
leeches tend to prove that the coelom and haemocoel are primitively
one series of spaces which have been gradually differentiated. The
facts of development, however, prove their distinctness, though those
same facts do not speak clearly as to the true nature of the blood
system. One view of the origin of the latter (largely based upon
observations upon the development of _Polygordius_) sees in the blood
system a persistent blastocoel. F. Vezhdovsky has lately seen reasons
for regarding the blood system as originating entirely from the
hypoblast by the secretion of fluid, the blood, from particular
intestinal cells and the consequent formation of spaces through
pressure, which become lined with these cells.

_Nephridia and Coelomoducts_.--The name "Nephridium" was originally
given by Sir E. Ray Lankester to the members of a series of tubes,
proved in some cases to be excretory in nature, which exist typically
to the number of a single pair in most of the segments of the
Chaetopod body, and open each by a ciliated orifice into the coelom on
the one hand, and by a pore on to the exterior of the body on the
other. In its earlier conception, this view embraced as homologous
organs (so far as the present group is concerned) not only the
nephridia of Oligochaeta and Hirudinea, which are obviously closely
similar, but the wide tubes with an intercellular lumen and large
funnels of certain Polychaeta, and (though with less assurance) the
gonad ducts in Oligochaeta and Hirudinea. The function of nitrogenous
excretion was not therefore a necessary part of the view--though it
may be pointed out that there are grounds for believing that the gonad
ducts are to some extent also organs of excretion (see below). Later,
the investigations of E. Meyer and E.S. Goodrich, endorsed by
Lankester, led to the opinion that under the general morphological
conception of "nephridium" were included two distinct sets of organs,
viz. nephridia and coelomoducts. The former (represented by, e.g. the
"segmental organs" of _Lumbricus_) have been asserted to be
"ultimately, though not always, actually traceable to the ectoderm";
the latter (represented by, e.g. the oviduct of _Lumbricus_) are parts
of the coelomic wall itself, which have grown out to the exterior. The
nephridia, in fact, on this view, are _ectodermic ingrowths_, the
coelomoducts _coelomic outgrowths_. The cavity of the former has
nothing to do with coelom. The cavity of the latter is coelom.

The embryological facts upon which this view has been based, however,
have been differently interpreted. According to C.O. Whitman the
entire nephridial system (in the leech _Clepsine_) is formed by the
differentiation of a continuous epiblastic band on each side. The
exact opposite is maintained by R.S. Bergh (for _Lumbricus_ and
_Criodrilus_), whose figures show a derivation of the entire
nephridium from mesoblast, and an absence of any connexion between
successive nephridia by any continuous band, epiblastic or
mesoblastic. A midway position is taken up by Wilson, who asserts the
mesoblastic formation of the funnel, but also asserts the presence of
a continuous band of epiblast from which certainly the terminal
vesicle of the nephridium, and doubtfully the glandular part of the
tube is derived. Vezhdovsky's figures of _Rhynchelmis_ agree with
those of Bergh in showing the backward growth of the nephridium from
the funnel cell. There are thus substantial reasons for believing that
the nephridium grows backwards from a funnel as does the coelomoduct.
It is therefore by no means certain that so profound a difference
embryologically can be asserted to exist between the excretory
nephridia and the ducts leading from the coelom to the exterior, which
are usually associated with the extrusion of the genital products
among the Chaetopoda.

There are, however, anatomical and histological differences to be seen
at any rate at the extremes between the undoubted nephridia of
Goodrich, Meyer and Lankester, and the coelomoducts of the same
authors.

A, Diagram of the nephridium of _Nereis diversicolor_.
B, Diagram of the nephridium of _Alciope_, into which opens the large
genital funnel (coelomostome).
C, Small portion of the nephridium of _Glycera siphonostoma_, showing
the canal cut through, and the solenocytes on the outer surface.
D, Optical section of a branch of the nephridium of _Nephthys
scolopendroides_.
c.s, Cut surface.
cst, Coelomostome.
f, Flagellum.
g.f, Genital funnel.
n, Neck of solenocyte.
n.c, Nephridial canal.
n.p, Nephridiopore.
nst, Nephridiostome.
nu, Nucleus of solenocyte.
s, Solenocytes.
t, Tube.]

I. _Nephridia_.--Excretory organs which are undisputed nephridia are
practically universal among the Oligochaeta, Hirudinea and
Archiannelida, and occur in many Polychaeta. Their total absence has
been asserted definitely only in _Paranais littoralis_. Usually these
organs are present to the number of a single pair per somite, and are
commonly present in the majority of the segments of the body, failing
often among the Oligochaeta in a varying number of the anterior
segments. They are considerably reduced in number in certain
Polychaeta. Essentially, a nephridium is a tube, generally very long
and much folded upon itself, composed of a string of cells placed end
to end in which the continuous lumen is excavated. Such cells are
termed "drain pipe" cells. Frequently the lumen is branched and may
form a complicated anastomosing network in these cells. Externally,
the nephridium opens by a straight part of the tube, which is often
very wide, and here the intracellular lumen becomes intercellular.
Rarely the nephridium does not communicate with the coelom; in such
cases the nephridium ends in a single cell, like the "flame cell" of a
Platyhelminth worm, in which there is a lumen blocked at the coelomic
end by a tuft of fine cilia projecting into the lumen. This is so with
_Aeolosoma_ (Vezhdovsky). The condition is interesting as a
persistence of the conditions obtaining in the provisional nephridia
of e.g. _Rhynchelmis_, which afterwards become by an enlargement and
opening up of the funnel the permanent nephridia of the adult worm. In
some Polychaets (e.g. _Glycera_, see fig. 2) there are many of these
flame cells to a single nephridium which are specialized in form, and
have been termed "solenocytes" (Goodrich). They are repeated in
_Polygordius_, and are exactly to be compared with similarly-placed
cells in the nephridia of _Amphioxus_.

More usually, and indeed in nearly every other case among the
Oligochaeta and Hirudinea, the coelomic aperture of the nephridium
consists of several cells, ciliated like the nephridium itself for a
greater or less extent, forming a funnel. The funnel varies greatly in
size and number of its component cells. There are so many differences
of detail that no line can be drawn between the one-celled funnel of
_Aeolosoma_ and the extraordinarily large and folded funnel of the
posterior nephridia in the Oligochaete _Thamnodrilus_. In the
last-mentioned worm the funnels of the anterior nephridia are small
and but few celled; it is only the nephridia in and behind the 17th
segment of the body which are particularly large and with a sinuous
margin, which recall the funnels of the gonad ducts (i.e.
coelomoducts).

Among the Polychaeta the nephridium of _Nereis_ (see fig. 2) is like
that of the Oligochaeta and Hirudinea in that the coiled glandular
tube has an intracellular duct which is ciliated in the same way in
parts. The Polychaeta, however, present us with another form of
nephridium seen, for example, in _Arenicola_, where a large funnel
leads into a short and wide excretory tube whose lumen is
intercellular. In the young stages of this worm which have been
investigated by W.B. Benham, the tube, though smaller, and with a but
little pronounced funnel, has still an intercellular duct. That these
organs in Polychaeta serve for the removal of the generative products
to the exterior is proved not only by the correspondence in number to
them of the gonads, but by actual observation of the generative
products in transit. This form of nephridia leads to the shorter but
essentially similar organs in the Polychaete _Sternaspis_, and to
those of the Echiuroidea (q.v.) and of the Gephyrea (q.v.).

Though the paired arrangement of the nephridia is the prevalent one in
the Chaetopoda, there are many examples, among the Oligochaeta, of
species and genera in which there are several, even many, nephridia in
each segment of the body, which may or may not be connected among
themselves, but have in any case separate orifices on to the exterior.

2. _Coelomoducis._--In this category are included (by Goodrich and
Lankester) the gonad ducts of the Oligochaeta, certain funnels without
any aperture to the exterior that have been detected in _Nereis_, &c.,
funnels with wide and short ducts attached to nephridia in other
Polychaeta, gonad ducts in the _Capitellidae_, the gonad ducts of the
leeches. In all these cases we have a duct which has a usually wide,
always intercellular, lumen, generally, if not always, ciliated, which
opens directly into the coelom on the one hand and on to the exterior
of the body on the other. These characters are plain in all the cases
cited, excepting only the leeches which will be considered separately.

There is not a great deal of difference between most of these
structures and true nephridia. It is not clear, for example, to which
category it is necessary to refer the excretory organs of _Arenicola_,
or _Polynoe_. Both series of organs consist essentially of a ciliated
tube leading from the coelom to the exterior. Both series of organs
grow back centrifugally from the funnel. In both the cavity originally
or immediately continuous with the coelom appears first in the funnel
and grows backwards. In some cases, e.g. oviducts of Oligochaeta,
sperm ducts of _Phreoryctes_, the coelomoducts occupy, like the
nephridia, two segments, the funnel opening into that in front of the
segment which carries the external pore. It is by no means certain
that a hard and fast line can be drawn between intra- and
intercellular lumina. Finally, in function there are some points of
likeness. The gonad ducts of _Lumbricus_, &c., must perform one
function of nephridia; they must convey to the exterior some of the
coelomic fluid with its disintegrated products of waste. There is no
possibility that sperm and ova can escape by these tubes not in
company with coelomic fluid. In the case of many Oligochaeta where
there is no vascular network surrounding the nephridium, this function
must be the chief one of those glands, the more elaborate process of
excretion taking place in the case of nephridia surrounded by a rich
plexus of blood capillaries. A consideration of the mode of
development and appearance of the coelomoducts that have thus far been
enumerated (with the possible exception of those of the leeches) seems
to show that there is a distinct though varying relation between them
and the nephridia. It has been shown that in _Tubifex_, and some other
aquatic Oligochaeta, the genital segments are at first provided with
nephridia, and that these disappear on the appearance of the
generative ducts, which are coelomoducts. In _Lumbricus_ the connexion
is a little closer; the funnel of the nephridium, in the segments in
which the funnels of the gonad ducts are to be developed, persists and
is continuous with the gonad duct funnels on their first appearance.
In the development of the Acanthodrilid earthworm _Octochaetus_ (F.E.
Beddard) the funnels of the pronephridia disappear except in the
genital segments, where they seem to be actually converted into the
genital funnels. At the least there is no doubt that the genital
funnels are developed precisely where the nephridial funnels formerly
existed. If the genital funnels are not wholly or partly formed out of
the nephridial funnels they have replaced them. In the genital
segments of _Eudrilus_ the nephridia are present, but the funnels have
not been found though they are obvious in other segments. Here also
the genital funnels have either replaced or been formed out of
nephridial funnels. In _Haplotaxis heterogyne_ (W.B. Benham) the
sperm ducts are hardly to be distinguished from nephridia; they are
sinuous tubes with an intra-cellular duct. But the funnel is large and
thus differs from the funnels of the nephridia in adjoining segments.
Here again the nephridial funnel seems to have been converted into or
certainly replaced by a secondarily developed funnel. This example is
similar to cases among the Polychaeta where a true nephridium is
provided with a large funnel, coelomostome, according to the
nomenclature of Lankester. The whole organ, having, as is thought but
not known, this double origin, is termed a nephromixium. The various
facts, however, seem to be susceptible of another interpretation. It
may be pointed out that the several examples described recall a
phenomenon which is not uncommon and is well known to anatomists. That
is the replacement of an organ by, sometimes coupled with its partial
conversion into, a similar or slightly different organ performing the
same or an analogous function. Thus the postcaval vein of the higher
vertebrata is partly a new structure altogether, and is partly formed
out of the pre-existing posterior cardinals. The more complete
replacements, such as the nephridia of the genital segment of
_Tubifex_ by a subsequently formed genital duct, may be compared with
the succession of the nesonephros to the pronephros in vertebrates,
and of the metanephros to the mesonephros in the higher vertebrates.
It might be well to term these structures, mostly serving as gonad
ducts, which have an undoubted resemblance to nephridia, and for the
most part an undoubted connexion with nephridia, "Nephrodinia," to
distinguish them from another category of "ducts" which are
communications between the coelom and the exterior, and which have no
relation whatever to nephridia or to the organs just discussed. For
these latter, the term coelomoducts might well be reserved. To this
category belong certain sacs and pouches in many, perhaps most, genera
of the Oligochaeta family, _Eudrilidae_, and possibly the gonad ducts
in the Hirudinea. As an example of the former it has been shown
(Beddard) that a large median sac in _Lybiodrilus_ is at first freely
open to the coelom, that it later becomes shut off from the same, that
it then acquires an external orifice, and, finally, that it encloses
the ovary or ovaries, between which and the exterior a passage is thus
effected. To this category will belong the oviducts in Teleostean
fishes and probably the gonad ducts in several groups of
invertebrates.

POLYCHAETA.--This group may be thus defined and the definition contrasted and compared with those of the other divisions of the Chaetopoda. Setae always present and often very large, much varied in form and very numerous, borne by the dorsal and ventral parapodia (when present). The prostomium and the segments generally often bear processes sensory and branchial. Eyes often present and comparatively complicated in structure. Clitellum not present as a definite organ, as in Oligochaeta. The anus is mostly terminal, and there are no anterior and posterior suckers. Nervous system often imbedded in the epidermis. Vascular system generally present forming a closed system of tubes. Alimentary canal rarely coiled, occasionally with glands which are simple caeca and sometimes serve as air reservoirs; jaws often present and an eversible pharynx. Nephridia sometimes of the type of those of the Oligochaeta; in other cases short, wide tubes with a large funnel serving also entirely or in part as gonad ducts. Frequently reduced in number of pairs; rarely (_Capitellidae_) more than one pair per segment. Gonads not so restricted in position as in Oligochaets, and often more abundant; the individuals usually unisexual. No specialized system of spermathecae, sperm reservoirs, and copulatory apparatus, as in Oligochaeta; development generally through a larval form; reproduction by budding also occurs. Marine (rarely fresh-water) in habit.

The Polychaeta contrast with the Oligochaeta by the great variety of outward form and by the frequency of specialization of different regions of the body. The head is always recognizable and much more conspicuous than in other Chaetopoda. As in the Oligochaeta the peristomial segment is often without setae, but this character is not by any means so constant as in the Oligochaeta. The prostomium bears often processes, both dorsal and ventral, which in the Sabellids are split into the circle of branchial plumes, which surround or nearly surround the mouth in those tube-dwelling Annelids. _Tomopteris_ is remarkable for the fact that the hammer-shaped prostomium has paired ventral processes each with a single seta. It is held, however, that these are a pair of parapodia which have shifted forwards. The presence of parapodia distinguish this from other groups of Chaetopoda. Typically, the parapodium consists of two processes of the body on each side, each of which bears a bundle of setae; these two divisions of the "limb" are termed respectively notopodium and neuropodium. The notopodium may be rudimentary or absent and the entire parapodium reduced to the merest ridge or even completely unrepresented. Naturally, it is among the free living forms that the parapodium is best developed, and least developed among the tubicolous Polychaeta. To each division of the parapodium belongs typically a long tentacle, the cirrus, which may be defective upon one or other of the notopodium or neuropodium, and may be developed into an arborescent gill or into a flat scale-like process, the elytron (in _Polynoe_, &c.). There are other gills developed in addition to those which represent the cirri.

_Setae_.--The setae of the Polychaeta are disposed in two bundles in
many genera, but in only one bundle in such forms as have no
notopodium (e.g. _Syllis_). In some genera the setae are in vertical
rows, and in certain _Capitellidae_ these rows so nearly meet that an
arrangement occurs reminiscent of the continuous circle of setae in
the perichaetous Oligochaeta. The setae vary much in form and are
often longer and stronger than in the Oligochaetes. Jointed setae and
very short hooks or "uncini" (see fig. 3) are among the most
remarkable forms. Simple bifid setae, such as those of Oligochaetes,
are also present in certain forms.

Among the burrowing and tubicolous forms it is not uncommon for the
body to be distinguishable into two or more regions; a "thorax," for
example, is sharply marked off from an "abdomen" in the Sabellids. In
these forms the bundles of setae are either capilliform or uncinate,
and the dorsal setae of the thorax are like the ventral setae of the
abdomen. It is a remarkable and newly-ascertained fact that in
regeneration (in _Potamilla_) the thorax is not replaced by the growth
of uninjured thoracic segments; but that the anterior segments of the
abdomen take on the same characters, the setae dropping out and being
replaced in accordance with the plan of the setae in the thorax of
uninjured worms. Among the Oligochaeta the sexually mature worm is
distinguished from the immature worm by the clitellum and by the
development of genital setae. Among the Polychaeta the sexual worm is
often more marked from the asexual form, so much so that these latter
have been placed in different species or even genera. The alteration
in form does not only affect structures used in generation; but the
form of the parapodia, &c., alter. There are even dimorphic forms
among the Syllids where the sexes are, as in many Polychaets,
separate.

_Nephridia_.--The nephridia of the Polychaeta have been generally
dealt with above in considering the nephridial system of the
Chaetopoda as a whole. They contrast with those of the Oligochaeta and
Hirudinea by reason of their frequently close association with the
gonads, the same organ sometimes serving the two functions of
excretion and conveyance of the ova and spermatozoa out of the body.
On the hypothesis that such a form as _Dinophilus_ (see Haplodrili)
has preserved the characters of the primitive Chaetopod more nearly
than any existing Polychaet or Oligochaet, it is clear that the
nephridia in the Oligochaeta have preserved the original features of
those organs more nearly than most Polychaeta. Thus _Nereis_ among the
latter worms, from the resemblance which its excretory system bears to
that of the Oligochaeta, may be made the starting-point of a series.
In this worm the paired nephridia exist in most of the segments of the
body, and their form (see fig. 2) is much like that of the nephridia
in the _Enchytraeidae_. The funnel, which is not large, appears to
open, as a rule at least, into the segment in front of that which
bears the external orifice. Quite independent of these are certain
large dorsally situate funnel-like folds of the coelomic epithelium,
ciliated, but of which no duct has been discovered leading to the
exterior. It is possible that we have here gonad ducts distinct from
nephridia which at the time of sexual maturity do open on to the
exterior.

In _Polynoe_ the nephridia are short tubes with a slightly folded
funnel whose lumen is intercellular, and this intercellular lumen is
characteristic of the Polychaetes as contrasted with leeches and
Oligochaetes. Among the Terebelloidea there is a remarkable
differentiation of the nephridia into two series. One set lies in
front of the diaphragm, which is the most anterior and complete
septum, the rest having disappeared or being much less developed. The
anterior nephridia, of which there are one to three pairs, contrast
with the posterior series by their small funnels and large size, the
posterior nephridia having a large funnel followed by a short tube. In
_Chaetozone setosa_ the anterior nephridia occupy five segments. There
is usually a gap between the two series, several segments being
without nephridia. It seems that the posterior nephridia are mainly
gonad ducts, and the gonads are developed in close association with
the funnels. The same arrangement is found in some other Polychaetes;
for instance, in _Sabellaria_ there is a single pair of large anterior
nephridia, which open by a common pore, followed after an interval by
large-funnelled and short nephridia. This differentiation is not,
however, peculiar to the Polychaetes; for in several Oligochaetes the
anterior nephridia are of large size, and opening as they do into the
buccal cavity clearly play a different function to those which follow.
In _Thamnodrilus_, as has been pointed out, there are two series of
nephridia which resemble those of the Terebelloidea in the different
sizes of their funnels. In _Lanice conchilega_ the posterior series of
nephridia are connected by a thick longitudinal duct, which seems to
be seen in its most reduced form in _Owenia_, where a duct on each
side runs in the epidermis, being in parts a groove, and receives one
short tubular nephridium only and occupies only one segment. This
connexion of successive nephridia (in _Lanice_) has its counterpart in
_Allolobophora, Lybiodrilus_, and apparently in the Lumbriculids
_Teleuscolex_ and _Styloscolex_, among the Oligochaeta. Among the
_Capitellidae_, which in several respects resemble the Oligochaeta,
wide and short gonad ducts coexist in the same segments with
nephridia, the latter being narrower and longer. It is noteworthy that
in this family only among the Polychaeta, the nephridia are not
restricted to a single pair in each segment; so that the older view
that the gonad ducts are metamorphosed nephridia is not at variance
with the anatomical facts which have been just stated.

_Alimentary Canal._--The alimentary canal of Polychaetes is usually a
straight tube running from the anterior mouth to the posterior anus.
But in some forms, e.g. _Sternaspis_, the gut is coiled. In others,
again, e.g. _Cobangia_, the anus is anterior and ventral. A gizzard is
present in a few forms. The buccal cavity is sometimes armed with
jaws. The oesophagus is provided often with caeca which in Syllids and
_Hesionidae_ have been found to contain air, and possibly therefore
perform the function of the fish's air-bladder. In other Polychaetes
one or more pairs of similar outgrowths are glandular. The intestine
is provided with numerous branched caeca in _Aphrodite_.

_Reproduction._--As is the case with the Oligochaeta, the Polychaeta
furnish examples of species which multiply asexually by budding. There
is a further resemblance between the two orders of Chaetopoda in that
this budding is not a general phenomenon, but confined to a few forms
only. Budding, in fact, among the Polychaetes is limited to the family
_Syllidae_. In the Oligochaetes it is only the families
_Aeolosomatidae_ and _Naididae_ that show the same phenomenon. It has
been mentioned that in the Nereids a sexual form occurs which differs
structurally from the asexual worms, and was originally placed in a
separate genus, _Heteronereis_; hence the name "Heteronereid" for the
sexual worm. In _Syllis_ there is also a "Heterosyllid" form in which
the gonads are limited to a posterior region of the body which is
further marked off from the anterior non-sexual segments by the
oak-like setae. In some Syllids this posterior region separates off
from the rest, producing a new head; thus a process of fission occurs
which has been termed schizogamy. A similar life history distinguishes
certain Sabellid worms, e.g. _Filigrana_. Among the Syllids this
simple state of affairs is further complicated. In _Autolytus_ there
is, to begin with, a conversion of the posterior half of the body to
form a sexual zooid. But before this separates off a number of other
zooids are formed from a zone of budding which appears between the two
first-formed individuals. Ultimately, a chain of sexual zooids is thus
formed. A given stock only produces zooids of one sex. In _Myrianida_
there is a further development of this process. The conversion of the
posterior end of the simple individual into a sexual region is
dispensed with; but from a preanal budding segment a series of sexual
buds are produced. The well-known Syllid, discovered during the voyage
of the "Challenger," shows a modification of this form of budding.
Here, however, the buds are lateral, though produced from a budding
zone, and they themselves produce other buds, so that a ramifying
colony is created.

Quite recently, another mode of budding has been described in
_Trypanosyllis gemmipara_, where a crowd of some fifty buds arising
symmetrically are produced at the tail end of the worm. In some
Syllids, such as _Pionosyllis gestans_, the ova are attached to the
body of the parent in a regular line, and develop in situ; this
process, which has been attributed to budding, is an "external
gestation," and occurs in a number of species.

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.