_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.
[Illustration: FIG 1.--A, side view of the head region of _Nereis
cultrifera_; B, dorsal view of the same.
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.
[Illustration: FIG. 2. (from Goodrich).
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.
[Illustration: Fig. 3.--a, Bristle of _Pionosyllis Malmgreni_; b, Hook
of _Terebella_.]
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_.
[Illustration: FIG. 4.--_Dasychone infracta_, Kr. (After Malmgren.)]
_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.
[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
adopted 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 groups. In both, the
nephridia are all alike; there are no jaws; the prostomium rarely has
processes. The body is often divisible into regions.
LITERATURE.--W.B. Benham, "Polychaeta" in _Cambridge Natural History_;
E. Claparede, _Annelides chetopodes du golfe de Naples_ (1868 and
1870); E. Ehlers, _Die Borstenwurmer_ (1868); H. Eisig, _Die
Capitelliden_ (Naples Monographs), and development of do. in _Mitth.
d. zool. Stat. Neapel_ (1898); W.C. M'Intosh, _"Challenger" Reports_
(1885); E.R. Lankester, Introductory Chapter in _A Treatise on
Zoology_; E.S. Goodrich, _Quart. Journ. Mic. Sci._ (1897-1900); E.
Meyer, _Mitth. d. zool. Stat. Neapel_ (1887, 1888), as well as
numerous other memoirs by the above and by J.T. Cunningham, de St
Joseph, A. Malaquin, A. Agassiz, A.T. Watson, Malmgren, Bobretsky and
A.F. Marion, E.A. Andrews, L.C. Cosmovici, R. Horst, W. Michaelsen, G.
Gilson, F. Buchanan, H. Levinsen, Joyeux-Laffuie, F.W. Gamble, &c.
OLIGOCHAETA.--As contrasted with the other subdivisions of the
Chaetopoda, the Oligochaeta may be thus defined. Setae very rarely
absent (genus _Achaeta_) and as a rule not so large or so numerous in
each segment as in the Polychaeta, and different in shape. Eyes rarely
present and then rudimentary. Prostomium generally small, sometimes
prolonged, but never bearing tentacles or processes. Appendages of body
reduced to branchiae, present only in four species, and to the ventral
copulatory appendages of _Alma_ and _Criodrilus_. Clitellum always
present, extending over two (many limicolous forms) to forty-five
segments (_Alma_). Segments of body numerous and not distinctive of
species, being irregular and not fixed in numbers. In terrestrial forms
dorsal pores are usually present; in aquatic forms a head pore only.
Anus nearly always terminal, rarely dorsal, at a little distance from
end of body. Suckers absent. Nervous system rarely (_Aeolosoma_) in
continuity with epidermis. Vascular system always present, forming a
closed system, more complicated in the larger forms than in the aquatic
genera. Several specially large contractile trunks in the anterior
segments uniting the dorsal and ventral vessels. Nephridia generally
paired, often very numerous in each segment, in the form of long,
much-coiled tubes with intracellular lumen. Gonads limited in number of
pairs, testes and ovaries always present in the same individual. Special
sacs developed from the intersegmental septa lodge the developing ova
and sperm. Special gonad ducts always present. Male ducts often open on
to exterior through a terminal chamber which is variously specialized,
and sometimes with a penis.
[Illustration:
FIG. 10.--Diagrams of various Earthworms, to illustrate external
characters. A, B, C, anterior segments from the ventral surface; D,
hinder end of body of _Urochaeta_.
A, _Lumbricus_: 9, 10, segments containing spermathecae, the orifices
of which are indicated; 14, segment bearing oviducal pores; 15,
segment bearing male pores; 32, 37, first and last segments of
clitellum.
B, _Acanthodrilus_: cp, orifices of spermathecae; [Female], oviducal
pores; [Male], male pores; on 17th and 19th segments are the apertures
of the atria.
C, _Perichaeta_: the spermathecal pores are between segments 6 and 7,
7 and 8, 8 and 9, the oviducal pores upon the 14th and the male pores
upon the 18th segment.
In all the figures the nephridial pores are indicated by dots and the
setae by strokes.]
Generative pores usually paired, sometimes single and median.
Spermathecae nearly always present. Alimentary canal straight, often
with appended glands of complicated or simpler structure; no jaws. Eggs
deposited in a cocoon after copulation. Development direct. Reproduction
by budding also occurs. Fresh-water (rarely marine) and terrestrial.
The Oligochaeta show a greater variety of size than any other group of
the Chaetopoda. They range from a millimetre or so (smaller species of
_Aeolosoma_) to 6 ft. or even rather more (_Microchaeta rappi_, &c.) in
length.
_Setae._--The setae, which are always absent from the peristomial
segment, are also sometimes absent from a greater number of the
anterior segments of the body, and have completely disappeared in
_Achaeta cameranoi._ When present they are either arranged in four
bundles of from one to ten or even more setae, or are disposed in
continuous lines completely encircling each segment of the body. This
latter arrangement characterizes many genera of the family
_Megascolicidae_ and one genus (_Periscolex_) of the
_Glossoscolicidae._ It has been shown (Bourne) that the "perichaetous"
condition is probably secondary, inasmuch as in worms which are, when
adult, "perichaetous" the setae develop in pairs so that the embryo
passes through a stage in which it has four bundles of setae, two to
each bundle, the prevalent condition in the group. Rarely there is an
irregular disposition of the setae which are not paired, though the
total number is eight to a segment (fig. 10), e.g. _Pontoscolex._ The
varying forms of the setae are illustrated in fig. 11.
[Illustration: FIG. 11.--Setae of _Oligochaeta_.
a, Penial seta of _Perichaeta ceylonica._
b, Extremity of penial seta of _Acanthodrilus_ (after Horst).
c, Seta of _Urochaeta_ (Perier).
d, Seta of _Lumbricus._
e, Seta of _Criodrilus._
f, g, Setae of _Bohemilla comata._
h, i, j, Setae of _Psammoryctes barbatus_ (f to j after Vezhdovsky).]
_Structure._--The body wall consists of an epidermis which secretes a
delicate cuticle and is only ciliated in _Aeolosoma_, and in that
genus only on the under surface of the prostomium. The epidermis
contains numerous groups of sense cells; beneath the epidermis there
is rarely (_Kynotus_) an extensive connective tissue dermis. Usually
the epidermis is immediately followed by the circular layer of
muscles, and this by the longitudinal coat. Beneath this again is a
distinct peritoneum lining the coelom, which appears to be wanting as
a special layer in some Polychaetes (Benham, Gilson). The muscular
layers are thinner in the aquatic forms, which possess only a single
row of longitudinal fibres, or (_Enchytracidae_) two layers. In the
earthworms, on the other hand, this coat is thick and composed of many
layers.
The clitellum consists of a thickening of the epidermis, and is of two
forms among the Oligochaeta. In the aquatic genera the epidermis comes
to consist entirely of glandular cells, which are, however, arranged
in a single layer. In the earthworms, on the other hand, the epidermis
becomes specialized into several layers of cells, all of which are
glandular. It is therefore obviously much thicker than the clitellum
in the limicolous forms. The position of the clitellum, which is
universal in occurrence, varies much as does the number of component
segments. As a rule--to which, however, there are exceptions--the
clitellum consists of two or three segments only in the small aquatic
Oligochaeta, while in the terrestrial forms it is as a general rule,
to which again there are exceptions, a more extensive, sometimes much
more extensive, region.
In the Oligochaeta there is a closer correspondence between external
metamerism and the divisions of the coelom than is apparent in some
Chaetopods. The external segments are usually definable by the setae;
and if the setae are absent, as in the anterior segments of several
_Geoscolicidae_, the nephridiopores indicate the segments; to each
segment corresponds internally a chamber of the coelom which is
separated from adjacent segments by transverse septa, which are only
unrecognizable in the genus _Aeolosoma_ and in the head region of
other Oligochaeta. In the latter case, the numerous bands of muscle
attaching the pharynx to the parietes have obliterated the regular
partition by means of septa.
_Nephridia_.--The nephridia in this group are invariably coiled tubes
with an intracellular lumen and nearly invariably open into the coelom
by a funnel. There are no renal organs with a wide intercellular
lumen, such as occur in the Polychaeta, nor is there ever any
permanent association between nephridia and ducts connected with the
evacuation of the generative products, such as occur in _Alciope_,
_Saccocirrus_, &c. In these points the Oligochaeta agree with the
Hirudinea. They also agree in the general structure of the nephridia.
It has been ascertained that the nephridia of Oligochaeta are preceded
in the embryo by a pair of delicate and sinuous tubes, also found in
the Hirudinea and Polychaeta, which are larval excretory organs. It is
not quite certain whether these are to be regarded as the remnant of
an earlier excretory system, replaced among the Oligochaeta by the
subsequently developed paired structures, or whether these "head
kidneys" are the first pair of nephridia precociously developed. The
former view has been extensively held, and it is supported by the fact
that in _Octochaetus_ the first segment of the body has a pair of
nephridia which is exactly like those which follow, and, like them,
persists. On the other hand, in most Oligochaeta the first segment has
in the adult no nephridium, and in the case of _Octochaetus_ the
existence of a "head kidney" antedating the subsequently developed
nephridia of the first and other segments has neither been seen nor
proved to be absent. In any case the nephridia which occupy the
segments of the body generally are first of all represented by paired
structures, the "pronephridia," in which the funnel is composed of but
one cell, which is flagellate. This stage has at any rate been
observed in _Rhynchelmis_ and _Lumbricus_ (in its widest sense) by
Vezhdovsky. It is further noticeable that in _Rhynchelmis_ the
covering of vesicular cells which clothes the drain-pipe cells of the
adult nephridium is cut off from the nephridial cells themselves and
is not a peritoneal layer surrounding the nephridium. Thus the
nephridia, in this case at least, are a part of the coelom and are not
shut off from it by a layer of peritoneum, as are other organs which
lie in it, e.g. the gut. A growth both of the funnel, which becomes
multicellular, and of the rest of the nephridium produces the adult
nephridia of the genera mentioned. The paired disposition of these
organs is the prevalent one among the Oligochaeta, and occurs in all
of twelve out of the thirteen families into which the group is
divided.
Among the _Megascolicidae_, however, which in number of genera and
species nearly equals the remaining families taken together, another
form of the excretory system occurs. In the genera _Pheretima,
Megascolex_, _Dichogaster_, &c., each segment contains a large number
of nephridia, which, on account of the fact that they are necessarily
smaller than the paired nephridia of e.g. _Lumbricus_, have been
termed micronephridia, as opposed to meganephridia; there is, however,
no essential difference in structure, though micronephridia are not
uncommonly (e.g. _Megascolides_, _Octochaetus_) unprovided with
funnels. It is disputed whether these micronephridia are or are not
connected together in each segment and from segment to segment. In any
case they have been shown in three genera to develop by the growth and
splitting into a series of original paired pronephridia. A complex
network, however, does occur in _Lybiodrilus_ and certain other
_Eudrilidae_, where the paired nephridia possess ducts leading to the
exterior which ramify and anastomose on the thickness of the body
wall. The network is, however, of the duct of the nephridium, possibly
ectodermic in origin, and does not affect the glandular tubes which
remain undivided and with one coelomic funnel each.
The Oligochaeta are the only Chaetopods in which undoubted nephridia
may possess a relationship with the alimentary canal. Thus, in
_Octochaetus multiporus_ a large nephridium opens anteriorly into the
buccal cavity, and numerous nephridia in the same worm evacuate their
contents into the rectum. The anteriorly-opening and usually very
large nephridia are not uncommon, and have been termed
"peptonephridia."
_Gonads and Gonad Ducts_.--The Oligochaeta agree with the leeches and
differ from most Polychaeta in that they are hermaphrodite. There is
no exception to this generalization. The gonads are, moreover, limited
and fixed in numbers, and are practically invariably attached to the
intersegmental septa, usually to the front septum of a segment, more
rarely to the posterior septum. The prevalent number of testes is one
pair in the aquatic genera and two pairs in earthworms. But there are
exceptions; thus a species of _Lamprodrilus_ has four pairs of testes.
The ovaries are more usually one pair, but two are sometimes present.
The segments occupied by the gonads are fixed, and are for earthworms
invariably X, XI, or one of them for the testes, and XIII for the
ovaries The position varies in the aquatic Oligochaeta. The
Oligochaeta contrast with the Polychaeta in the general presence of
outgrowths of the septa in the genital segments, which are either
close to, or actually involve, the gonads, and into which may also
open the funnels of the gonad ducts. These sacs contain the developing
sperm cells or eggs, and are with very few exceptions universal in
the group. The testes are more commonly thus involved than are the
ovaries. It is indeed only among the _Eudrilidae_ that the enclosure
of the ovaries in septal sacs is at all general. Recently the same
thing has been recorded in a few species of _Pheretima_ (=
_Perichaeta_), but details are as yet wanting. We can thus speak in
these worms of _gonocoels_, i.e. coelomic cavities connected only with
the generative system. These cavities communicate with the exterior
through the gonad ducts, which have nothing to do with them, but whose
coelomic funnels are taken up by them in the course of their growth.
There are, however, in the _Eudrilidae_, as already mentioned, sacs
envolving the ovaries which bore their own way to the exterior, and
thus may be termed coelomoducts. These sacs are dealt with later under
the description of the spermathecae, which function they appear to
perform. The gonad ducts are male and female, and open opposite to or,
rarely, alongside of the gonads, whose products they convey to the
exterior. The oviducts are always short trumpet-shaped tubes and are
sometimes reduced (_Enchytraeidae_) to merely the external orifices.
It is possible, however, that those oviducts belong to a separate
morphological category, more comparable to the dorsal pores and to
abdominal pores in some fishes. The sperm ducts are usually longer
than the oviducts; but in Limicolae both series of tubes opening by
the funnel into one segment and on to the exterior in the following
segment. While the oviducts always open directly on to the exterior,
it is the rule for the sperm ducts to open on to the exterior near to
or through certain terminal chambers, which have been variously termed
atrium and prostate, or spermiducal gland. The distal extremity of
this apparatus is sometimes eversible as a penis. Associated with
these glands are frequently to be found bundles or pairs of long and
variously modified setae which are termed penial setae, to distinguish
them from other setae sometimes but not always associated with rather
similar glands which are found anteriorly to these, and often in the
immediate neighbourhood of the spermathecae; the latter are spoken of
as genital setae.
[Illustration: FIG. 12.--Female reproductive system of
_Heliodrilus_.--XI-XIV, eleventh to fourteenth segments, sperm,
spermatheca; sp.o, its external orifice; sp.sac, spermathecal sac; ov,
sac containing ovary; r.o, egg sac; od, oviduct.]
_Spermathecae._--These structures appear to be absolutely distinctive
of the Oligochaeta, unless the sacs which contain sperm and open in
common with the nephridia of _Saccocirrus_ (see HAPLODRILI) are
similar. Spermathecae are generally present in the Oligochaeta and are
absent only in comparatively few genera and species. Their position
varies, but is constant for the species, and they are rarely found
behind the gonads. They are essentially spherical, pear-shaped or oval
sacs opening on to the exterior but closed at the coelomic end. In a
few _Enchytraeidae_ and _Lumbriculidae_ the spermathecae open at the
distal extremity into the oesophagus, which is a fact difficult of
explanation. Among the aquatic Oligochaeta and many earthworms (the
families _Lunibricidae_, _Geoscolicidae_ and a few other genera) the
spermathecae are simple structures, as has been described. In the
majority of the _Megascolicidae_ each sac is provided with one or more
diverticula, tubular or oval in form, of a slightly different
histological character in the lining epithelium, and in them is
invariably lodged the sperm.
The spermathecae are usually paired structures, one pair to each of
the segments where they occur. In many _Geoscolicidae_, however, and
certain _Lumbricidae_ and _Perichaetidae_, there are several, even a
large number, of pairs of very small spermathecae to each of the
segments which contain them.
In the _Eudrilidae_ there are spermathecae of different morphological
value. In figs. 12 and 13 are shown the spermathecae of the genera
_Hyperiodrilus_ and _Heliodrilus_, which are simple sacs ending
blindly as in other earthworms, but of which there is only one median
opening in the thirteenth segment or in the eleventh. In _Heliodrilus_
the blind extremity of the spermatheca is enclosed in a coelomic sac
which is in connexion with the sacs envolving the ovaries and
oviducts. In _Hyperiodrilus_ the whole spermatheca is thus included in
a corresponding sac, which is of great extent. In such other genera of
the family as have been examined, the true spermatheca has entirely
disappeared, and the sac which contains it in _Hyperiodrilus_ alone
remains. This sac has been already referred to as a coelomoduct. Its
orifice on to the exterior is formed by an involution (as it appears)
of the epidermis, and that it performs the function of a spermatheca
is shown by its containing spermatozoa, or, in _Stuhlmannia_, a
spermatophore. In _Polytoreutus_, also, spermatophores have been found
in these spermathecal sacs. We have thus the replacement of a
spermatheca, corresponding to those of the remaining families of
Oligochaeta, and derived, as is believed, from the epidermis, by a
structure performing the same function, but derived from the
mesoblastic tissues, and with a cavity which is coelom.
_Alimentary Canal._--The alimentary canal is always a straight tube,
and the anus, save in the genera _Criodrilus_ and _Dero_, is
completely terminal. A buccal cavity, a pharynx, an oesophagus and an
intestine are always distinguishable. Commonly among the terrestrial
forms there is a gizzard, or two gizzards, or a larger number, in the
oesophageal region. There is no armed protrusible pharynx, such as
exists in some other Chaetopods. This may be associated with
mud-eating habits; but it is not wholly certain that this is the case;
for in _Chaetogaster_ and _Agriodrilus_, which are predaceous worms,
there is no protrusible pharynx, though in the latter the oesophagus
is thickened through its extent with muscular fibres. The oesophagus
is often furnished with glandular diverticula, the "glands of Morren,"
which are often of complex structure through the folding of their
walls. Among the purely aquatic families such structures are very
rare, and are represented by two caeca in the genus _Limnodriloides_.
It is a remarkable fact, not yet understood, that in certain
_Enchytraeidae_ and _Lumbriculidae_ the spermathecae open into the
oesophagus as well as on to the exterior. The only comparable fact
among other worms is the Laurer's canal or genito-intestinal canal in
the Trematoda. The intestine is usually in the higher forms provided
with a typhlosole, in which, in _Pontoscolex_, runs a ciliated canal
or canals communicating with the intestine. It is possible that this
represents the syphon or supplementary intestine of _Capitellidae_,
which has been shown to develop as a grooving of the intestine
ultimately cut off from it. The intestine has a pair of caeca or two
or three pairs (but all lie in one segment) in the genus _Pheretima_
and in one species of _Rhinodrilus_. In _Typhoeus_ and _Megascolex_
there are complex glands appended to the intestine.
[Illustration: FIG. 13.--Female reproductive system of
_Hyperiodrilus_.--XIII, XIV, thirteenth and fourteenth segments.