Chapter VII: Part 7

_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.

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.

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.

_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.

sp, Spermatheca.
sp', Spermathecal sac involving the last.
ov, Ovary.
r.o, Egg sac.
od, Oviduct.]

In _Benhamia caecifera_ and at least one other earthworm there are
numerous caeca, one pair to each segment.

_Classification._--The classifications of Adolf Eduard, Grube and
Claparede separated into two subdivisions the aquatic and the
terrestrial forms. This scheme, opposed by many, has been reinstated
by Sedgwick. The chief difficulty in this scheme is offered by the
Moniligastridae, which in some degree combine the characters of both
the suborders, into neither of which will they fit accurately. The
following arrangement is a compromise:--

Group I. _Aphaneura._--This group is referred by A. Sedgwick to the
Archiannelida. It is, however, though doubtless near to the base of
the Oligochaetous series, most nearly allied in the reproductive
system to the Oligochaeta. It contains but one family,
_Aeolosomatidae_. There are three pairs of spermathecae situated in
segments III-V, a testis in V and an ovary in VI. There are a
clitellum and sperm ducts which though like nephridia have a larger
funnel and a less complexly wound duct. This family consists of only
one well-known genus, _Aeolosoma_, which contains several species.
They are minute worms with coloured oil drops (green, olive green or
orange) contained in the epidermis. The nervous system is embedded in
the epidermis, and the pairs of ganglia are separated as in _Serpula_,
&c.; each pair has a longish commissure between its two ganglia. The
intersegmental septa are absent save for the division of the first
segment. The large prostomium is ciliated ventrally. The setae are
either entirely capillary or there are in addition some sigmoid setae
even with bifid free extremities. This genus also propagates
asexually, like _Ctenodrilus_, which may possibly belong to the same
family. Asexual reproduction universal.

Group II. _Limicolae._--With a few exceptions the Limicolae are, as
the name denotes, aquatic in habit. They are small to moderate-sized
Oligochaeta, with a smaller number of segments than in the Terricolae.
The alimentary canal is simple and a gizzard or oesophageal
diverticula rarely developed. The vascular system is simple with as a
rule direct communication between dorsal and ventral vessels in each
segment. Nerve cord lies in coelom; brain in first segment or
prostomium in many forms. Clitellum generally only two or three
segments and more anterior in position than in Terricolae. Nephridia
always paired and without plexus of blood capillaries. Spermatheca
rarely with diverticula; sperm ducts as a rule occupying two segments
only, usually opening by means of an atrium. Sperm sacs generally
occupying a good many segments and with simple interior undivided by a
network of trabeculae. Ova large and with much yolk. Asexual
reproduction only in Naids. Egg sacs as large or nearly so as sperm
sacs. Testes and ovaries always free. The following families
constitute the group, viz. _Naididae_, _Enchytraeidae_, _Tubificidae_,
_Lumbriculidae_, _Phreoryctidae_, _Phreodrilidae_, _Alluroididae_, the
latter possibly not referable to this group.

Group III. _Moniligastres._--Moderate-sized to very large Oligochaeta,
terrestrial in habit, with the appearance of Terricolae. Generative
organs anterior in position as in Limicolae. Sperm ducts and atria as
in Limicolae; egg sacs large; body wall thick; vascular system and
nephridia as in Terricolae. Only one family, _Moniligastridae_.

Group IV. _Terricolae._--Earthworms, rarely aquatic in habit. Of small
to very large size. Clitellum commonly extensive and more posterior in
position than in other groups. Vascular system complicated without
regular connexion between dorsal and ventral vessels, except in
anterior segments. Nephridia as a rule with abundant vascular supply.
Testes, and occasionally ovaries, enclosed in sacs. Sperm sacs
generally limited to one or two segments with interior subdivided by
trabeculae. Sperm ducts traverse several segments on their way to
exterior. They open in common with, or near to, or, more rarely, into,
glands which are not certainly comparable to the atria of the
Limicolae. Egg sacs minute and functionless(?). Eggs minute with
little yolk. Nephridia sometimes very numerous in each segment.
Spermathecae often with diverticula.

Earthworms are divided into the following families, viz.
_Megascolicidae_, _Geoscolicidae_, _Eudrilidae_, _Lumbricidae_.

As an appendix to the Oligochaeta, and possibly referable to that
group, though their systematic position cannot at present be
determined with certainty, are to be placed the _Bdellodrilidae_
(_Discodrilidae_ auct.), which are small parasites upon crayfish.
These worms lay cocoons like the Oligochaeta and leeches, and where
they depart from the structure of the Oligochaeta agree with that of
leeches. The body is composed of a small and limited number of
segments (not more than fourteen), and there is a sucker at each end
of the body. There are no setae and apparently only two pairs of
nephridia, of which the anterior pair open commonly by a common pore
on the third segment after the head, whose segments have not been
accurately enumerated. The intervening segments contain the genitalia,
which are on the Oligochaeta plan in that the gonads are independent
of their ducts and that there are special spermathecae, one pair. The
male ducts are either one pair or two pairs, which open by a common
and complicated efferent terminal apparatus furnished with a
protrusible penis. The ganglia are crowded at the posterior end of the
body as in leeches, and there is much tendency to the obliteration of
the coelom as in that group. _Pterodrilus_ and _Cirrodrilus_ bear a
few, or circles of, external processes which may be branchiae;
_Bdellodrilus_ and _Astacobdella_ have none. The vascular system is as
in the lower Oligochaeta. There are two chitinous jaws in the buccal
cavity, a dorsal and a ventral, which are of specially complicated
structure in _Cirrodrilus_.

LITERATURE.--F.E. Beddard, _A Monograph of the Oligochaeta_ (Oxford,
1895), also _Quart. Journ. Micr. Sci._, 1886-1895, and _Proc. Zool.
Soc._, 1885-1906; W.B. Benham, _Quart. Journ. Micr. Sci._, 1886-1905;
W. Michaelsen, "Oligochaeta" in _Das Tierreich_, 1900, and _Mitth.
Mus._ (Hamburg, 1890-1906); A.G. Bourne, _Quart. Journ. Micr. Sci._,
1894; H.J. Moore, _Journ. Morph._, 1895; F. Vezhdovsky, _System d.
Oligochaeten_ (Prague, 1884), and _Entwicklungsgeschichtliche
Untersuchungen_; and numerous papers by the above and by G. Eisen, E.
Perrier, D. Rosa, R. Horst, L. Cognetti, U. Pierantoni, W. Baldwin
Spencer, H. Ude, &c., and embryological memoirs by R.S. Bergh, E.B.
Wilson, N. Kleinenberg, &c.

HIRUDINEA.--The leeches are more particularly to be compared with the Oligochaeta, and the following definition embraces the main features in which they agree and disagree with that group. Setae are only present in the genus _Acanthobdella_. Eyes are present, but hardly so complex as in certain genera of Polychaetes. The appendages of the body are reduced to branchiae, present in certain forms. A clitellum is present. The segments of body are few (not more than thirty-four) and fixed in number. The anus is dorsal. One or two (anterior and posterior) suckers always present. Nervous system always in coelom. Coelom generally reduced to a system of tubes, sometimes communicating with vascular system; in _Acanthobdella_ and _Ozobranchus_ a series of metamerically arranged chambers as in Oligochaeta. Nephridia always paired, rarely (_Pontobdella_) forming a network communicating from segment to segment; lumen of nephridia always intracellular, funnels pervious or impervious. Alimentary canal sometimes with protrusible proboscis; never with gizzard or oesophageal glands; intestine with caeca as a rule. Jaws often present. Testes several pairs, rarely one pair, continuous with sperm ducts; ovaries, one pair, continuous with oviducts; generative pores single and median. No separate spermathecae or septal chambers for the development of the ova and sperm. Eggs deposited in a cocoon. Development direct. No asexual generation. Fresh-water, marine and terrestrial. Parasitic or carnivorous.

In external characters the Hirudinea are unmistakable and not to be
confused with other Annelids, except perhaps with the
_Bdellodrilidae_, which resemble them in certain particulars. The
absence of setae--save in _Acanthobdella_, where five of the anterior
segments possess each four pairs of setae with reserve setae placed
close behind them (fig. 14), and the presence of an anterior and
posterior sucker, produce a looping mode of progression similar to
that of a Geometrid larva. The absence of setae and the great
secondary annulation render the mapping of the segments a subject of
some difficulty. The most reliable test appears to be the nerve
ganglia, which are more distinct from the intervening connectives than
in other Annelids.

In the middle of the body, where the limits of the somites can be
checked by a comparison with the arrangement of the nephridia and the
gonads, and where the ganglia are quite distinct and separated by long
connectives, each ganglion is seen to consist of six masses of cells
enclosed by capsules and to give off three nerves on each side. This
corresponds to the usual presence (in the _Rhynchobdellidae_) of three
annuli to each segment. Anteriorly and posteriorly separate ganglia
have fused. The brain consists not only of a group of six capsules
corresponding to the archicerebrum of the Oligochaeta, but of a
further mass of cells surrounding and existing below the alimentary
canal, which can be analysed into five or six more separate ganglia.
The whole mass lies in the seventh or eighth segment. At the posterior
end of the body there are likewise seven separate ganglia partially
fused to form a single ganglionic mass, which innervates the segments
lying behind the anus and corresponding to the posterior sucker. So
that a leech in which only twenty-seven segments are apparent by the
enumeration of the annuli, separate ganglia, nephridia, lines of
sensillae upon the body, really possesses an additional seven lying
behind that which is apparently the last of the series and crowded
together into a minute space. The annuli into which segments are
externally divided are so deeply incised as to render it impossible to
distinguish, as can be readily done in the Oligochaeta as a rule, the
limits of an annulus from that of a true segment. As remarked, the
prevalent number of annuli to a segment is three in the
_Rhynchobdellidae_. But in that group (_Cystobranchus_) there may be
as many as eight annuli. In the _Gnathobdellidae_ the prevailing
number of annuli to a segment is five; but here again the number is
often increased, and _Trocheta_ has no less than eleven. The reason
for this excessive annulation has been seen in the limited number of
segments (thirty-four) of which the body is composed, which are laid
down early and do not increase. In the Oligochaeta, on the other hand,
there is growth of new segments. It is important to notice that the
metameric plan of growth of Chaetopods is still preserved.

The nephridia are like those of the Oligochaeta in general structure;
that is to say, they consist of drain-pipe cells which are placed end
to end and are perforated by their duct. The internal funnel varies in
the same way as in the Oligochaeta in the number of cells which form
it. In _Clepsine_ (_Glossiphonia_) there are only three cells, and in
_Nephelis_ five to eight cells. In _Hirudo_ the funnel is not pervious
and is composed of a large number of cells. Externally, the nephridium
opens by a vesicle, as in many Oligochaetes whose lumen is
intercellular. In _Pontobdella_ and _Branchellion_ the nephridia form
a network extending from segment to segment, but there is only one
pair of funnels in each segment. Slight differences in form have been
noted between nephridia of different segments; but the Hirudinea do
not show the marked differentiation that is to be seen in some other
Chaetopods; nor do the nephridia ever acquire any relations to the
alimentary canal.

c, Coelom.
c.ch, Coelomic epithelium (yellow-cells).
cg, Glandular cells.
cl, Muscle cells of lateral line.
cp, Pigment cells.
ep, Ectoderm.
g, Nerve cord.
m, Intestine.
mc, Circular muscle.
ml, Longitudinal muscle.
vd, Dorsal vessel.
vv, Ventral vessel.]

_Coelom._--The coelom of the Hirudinea differs in most genera from
that of the Oligochaeta and Polychaeta. The difference is that it is
broken up into a complex sinus system. The least modified type is
shown by _Acanthobdella_, a leech, parasitic upon fishes, in which
transverse sections (see figs. 15 and 16) show the gut, the nervous
system, &c., lying in a spacious chamber which is the coelom. This
coelom is lined by peritoneal cells and is divided into a series of
metameres by septa which correspond to the segmentation of the body,
the arrangement being thus precisely like that of typical Chaetopoda.
Moreover, upon the intestine the coelomic cells are modified into
chloragogen cells. In _Acanthobdella_ the testes are, however, not
contained in the general coelom, and the nephridia lie in the septa.
It is remarkable, in view of the spaciousness of the coelom, that the
funnels of the latter have not been seen. _Ozobranchus_ possesses a
coelom which is less typically chaetopodous than that of
_Acanthobdella_, but more so than in other leeches. There is a
spacious cavity surrounding the gut and containing also blood-vessels,
and to some extent the generative organs, and the nervous cord.
Furthermore, in the mid region of the body this coelom is broken up by
metamerically arranged septa, as in _Acanthobdella_. These septa are,
however, rather incomplete and are not fastened to the gut; and, as in
_Acanthobdella_, the nephridia are embedded in them. In addition to
the median lacuna there are two lateral lacunae, one upon each side.
These regions of the coelom end at the ends of the body and
communicate with each other by means of a branched system of coelomic
sinuses, which are in places very fine tubes. Neither in this genus
nor in the last is there any communication between coelom and vascular
system. In _Clepsine_ (_Glossiphonia_) there is a further breaking up
of the coelom. The median lacuna no longer exists, but is represented
by a dorsal and ventral sinus. The former lodges the dorsal, the
latter the ventral, blood-vessel. The gut has no coelomic space
surrounding it. A complex network places these sinuses and the
lateral sinuses in communication. Here also the blood system has no
communication with the sinus system of the coelom. In _Hirudo_ and the
_Gnathobdellidae_ there is only one system of cavities which consist
of four principal longitudinal trunks, of which the two lateral are
contractile, which communicate with a network ramifying everywhere,
even among the cells of the epidermis. The network is partly formed
out of pigmented cells which are excavated and join to form tubes, the
so-called botryoidal tissue, not found among the _Rhynchobdellidae_ at
all. It seems clear from the recent investigations of A.G. Bourne and
E.S. Goodrich that the vascular system and the coelom are in
communication (as in vertebrates by means of the lymph system). On the
other hand, it has been held that in these leeches there is no
vascular system at all and that the entire system of spaces is coelom.
In favour of regarding the vascular system as totally absent, is the
fact that the median coelomic channels contain no dorsal and ventral
vessel. In favour of seeing in the lateral trunks and their branches a
vascular system, is the contractility of the former, and the fact of
the intrusion of the latter into the epidermis, matched among the
Oligochaeta, where undoubted blood capillaries perforate the
epidermis. A further fact must be considered in deciding this
question, which is the discovery of ramifying coelomic tubes,
approaching close to, but not entering, the epidermis in the
Polychaete _Arenicola_. These tubes are lined by flattened epithelium
and often contain blood capillaries; they communicate with the coelom
and are to be regarded as prolongation of it into the thickness of the
body wall.

_Gonads and Gonad Ducts._--The gonads and their ducts in the Hirudinea
invariably form a closed system of cavities entirely shut off from the
coelom in which they lie. There is thus a broad resemblance to the
_Eudrilidae_, to which group of Oligochaeta the Hirudinea are further
akin by reason of the invariably unpaired condition of the generative
apertures, and the existence of a copulatory apparatus (both of which
characters, however, are present occasionally in other Oligochaeta).

The testes are more numerous than the ovaries, of which latter there
are never more than one pair. The testes vary in numbers of pairs.
Four (_Ozobranchus_) to six (_Glossiphonia_) or ten (_Philaemon_) are
common numbers. In _Acanthobdella_, however, the testes of each side
of the body have grown together to form a continuous band, which
extends in front of external pore. Each testis communicates by means
of an efferent duct with a common collecting duct of its side of the
body, which opens on to the exterior by means of a protrusible penis,
and to which is sometimes appended a seminal vesicle. The efferent
ducts are ciliated, and there is a patch of cilia at the point where
they communicate with the cavity of each testis. The ovaries are more
extensive in some forms (e.g. _Ozobranchus_) than in others, where
they are small rounded bodies. The two ducts continuous with the
gonads open by a common vagina on to the exterior behind the male
pores. This "vagina" is sometimes of exaggerated size. Thus, in
_Philaemon pungens_ (Lambert) it has the form of a large sac, into
which open by a single orifice the conjoined oviducts. From this
vagina arises a narrow duct leading to the exterior. In _Ozobranchus_
the structures in question are still more complicated. The two long
ovarian sacs communicate with each other by a transverse bridge before
uniting to form the terminal canal. Into each ovarian sac behind the
transverse junction opens a slender tube, which is greatly coiled,
and, in its turn, opens into a spherical "spermathecal sac." From this
an equally slender tube proceeds, which joins its fellow of the
opposite side, and the two form a thick, walled tube, which opens on
to the exterior within the bursa copulatrix through which the penis
protrudes. These two last-mentioned types show features which can be,
as it seems, matched in the Eudrilidae.

The gonads develop (O. Burger) in coelomic spaces close to nephridial
funnels, which have, however, no relation to the gonad ducts. The
ovaries are solid bodies, of which the outer layer becomes separated
from the plug of cells lying within; thus a cavity is formed which is
clearly coelom. This cavity and its walls becomes prolonged to form
the oviducts. A stage exactly comparable to the stage in the leeches,
where the ovary is surrounded by a closed sac, has been observed in
_Eudrilus_. In this Annelid later the sac in question joins its
fellow, passing beneath the nerve cord exactly as in the leech, and
also grows out to reach the exterior. The sole difference is therefore
that in _Eudrilus_ the ovarian sac gives rise to a tube which
bifurcates, one branch meeting a corresponding branch of the other
ovary of the pair, while the second branch reaches the exterior. In
the leech the two branches are fused into one. We have here clearly a
case of a true coelomoduct performing the function of an oviduct in
both leeches and _Eudrilidae_. The facts just referred to suggest
further comparisons between the Hirudinea and _Eudrilidae_. The large
sacs which have been termed vagina are suggestive of the large
coelomic spermathecae in Eudrilids, a comparison which needs, however,
embryological data, not at present forthcoming, for its justification.
It is at least clear that in _Ozobranchus_ this comparison is
justifiable; but only probable, or perhaps possible, in the case of
_Philaemon_. In the former, the duct, leading from the ovarian sac,
and swelling along its course into the spherical sac, the
"spermatheca," is highly suggestive of the oviduct and receptaculum of
the _Eudrilidae_.

The testes during development become hollowed out and are prolonged
into the vasa efferentia. These ducts therefore have not their exact
counterparts in the Oligochaeta, unless we are to assume that they
collectively are represented by the seminal vesicles of earthworms and
the vasa deferentia. It is to be noted that the Hirudinea differ from
the Oligochaeta in that the male pore is in advance of the gonads
(except in _Acanthobdella_, which here, as in so many points,
approximates to the Oligochaeta), whereas in Oligochaeta that pore is
behind the gonads (again with an exception, _Allurus_).

_Classification_.--The Hirudinea may be divided into three families:--

(i.) _Rhynchobdellidae_.--A protrusible proboscis exists, but there
are no jaws. The blood is colourless. _Pontobdella_, _Glossiphonia_,
&c.

(ii.) _Gnathobdellidae_.--A proboscis absent, but jaws usually
present. Blood coloured red with haemoglobin. _Hirudo_, _Nephelis_,
&c.

(iii.) _Acanthobdellidae_.--Proboscis present, but short. Paired setae
of Oligochaetous pattern present in anterior segments. Blood red.
_Acanthobdella_.

LITERATURE.--A.O. Kovalevsky, _Bull. Imp. Sci._ (St Petersburg,
November 1896) (_Acanthobdella_); A.G. Bourne, _Quart. Journ. Micr.
Sci._, 1884; A. Oka, _Zeitschr. wiss. Zool._, 1894; E.S. Goodrich,
_Quart. Journ. Micr. Sci._, 1899; W.E. Castle, _Bull. Mus. Comp.
Zool._, 1900; A.M. Lambert, _Proc. Roy. Soc._ (Victoria, 1897); C.O.
Whitman, _Journ. Morph._, 1889 and 1891; O. Burger, _Zeitschr. wiss.
Zool._, 1902, and other memoirs by the above, and by St V. Apathy, R.
Blanchard, H. Bolsius, A. Dendy, R.S. Bergh, &c. (F. E. B.)

CHAETOSOMATIDA, a small group of minute, free-living, aquatic organisms which are usually placed as an annex to the Nematoda. Indeed Mechnikov, to whom we owe much of our knowledge of these forms, calls them "creeping Nematoda." They are usually found amongst seaweed in temperate seas, but they are probably widely distributed; some are fresh-water. The genus _Chaetosoma_, with the two species _Ch. claparedii_ and _Ch. ophicephalum_ and the genus _Tristicochaeta_, have swollen heads. The third genus _Rhabdogaster_ has no such distinct head, though the body may be swollen anteriorly. The mouth is terminal and anterior and surrounded by a ring of spicules or a half-ring of hooks. Scattered hairs cover the body. Just in front of the anus there is in _Chaetosoma_ a double, and in _Tristicochaeta_ a triple row of about fifteen stout cylindrical projections upon which the animals creep. The females are a little larger than the males; in _Ch. claparedii_ the former attain a length of 1.5 mm., the latter of 1.12 mm. The mouth opens into an oesophagus which passes into an intestine; this opens by a ventral anus situated a little in front of the posterior end. The testis is single, and its duct opens with the anus, and is provided with a couple of spicules. The ovary is double, and the oviducts open by a median ventral pore about the middle of the body; in this region there is a second swelling both in _Chaetosoma_ and in _Rhabdogaster_. The last-named form is in the female 0.36 mm. in length. In it the hairs are confined to the dorsal middle line and the creeping setae are hooked, of a finer structure than in _Chaetosoma_, and situated so far forward that the vagina opens amongst them. _Ch. ophicephalum_ has been taken in the English Channel.

Mature female of _Chaetosoma daparedii_, (From Mechnikov.) a, Oesophagus; b, intestine; c, anus; d, ovary; e, generative pore; f, ventral bristles.]

See E. Mechnikov, _Zeitschr. wiss. Zool._ xvii., 1867, p. 537;
Panceri, _Atti Acc. Napoli_, vii., 1878, p. 7. (A. E. S.)

CHAFER, a word used in modern speech to distinguish the beetles of the family _Scarabaeidae_, and more especially those species which feed on leaves in the adult state. The word is derived from the O. Eng. _ceafor_, and it is interesting to note that the cognate Ger. _Kafer_ is applied to beetles of all kinds. For the characters of the _Scarabaeidae_ see COLEOPTERA. This family includes a large number of beetles, some of which feed on dung and others on vegetable tissues. The cockchafers and their near allies belong to the subfamily _Melolonthinae_, and the rose-chafers to the _Cetoniinae_; in both the beetles eat leaves, and their grubs spend a long life underground devouring roots. In Britain the Melolonthines that are usually noted as injurious are the two species of cockchafer (_Melolontha vulgaris_ and _M. hippocastani_), large heavy beetles with black pubescent pro-thorax, brown elytra and an elongated pointed tail-process; the summer-chafer (_Rhizotrogus solstitialis_), a smaller pale brown chafer; and the still smaller garden-chafer or "cocker-bundy" (_Phyllopertha horticola_), which has a dark green pro-thorax and brown elytra. Of the Cetoniines, the beautiful metallic green rose-chafer, _Cetonia aurata_, sometimes causes damage, especially in gardens. The larvae of the chafers are heavy, soft-skinned grubs, with hard brown heads provided with powerful mandibles, three pairs of well-developed legs, and a swollen abdomen. As they grow, the larvae become strongly flexed towards the ventral surface, and lie curled up in their earthen cells, feeding on roots. The larval life lasts several years, and in hard frosts the grubs go deep down away from the surface. Pupation takes place in the autumn, and though the perfect insect emerges from the cuticle very soon afterwards, it remains in its underground cell for several months, not making its way to the upper air until the ensuing summer. After pairing, the female crawls down into the soil to lay her eggs. The grubs of chafers, when turned up by the plough, are greedily devoured by poultry, pigs and various wild birds. When the beetles become so numerous as to call for destruction, they are usually shaken off the trees where they rest on to sheets or tarred boards. On the continent of Europe chafers are far more numerous than in the United Kingdom, and the rural governments in France give rewards for their destruction. D. Sharp states that in the department of Seine-inferieure 867,173,000 cockchafers and 647,000,000 larvae were killed in the four years preceding 1870.

The anatomy of _Melolontha_ is very fully described in a classical
memoir by H.E. Strauss-Durckheim (Paris, 1828). (G. H. C.)

CHAFF (from the A.S. _ceaf_, allied to the O. High Ger. _cheva_, a husk or pod), the husks left after threshing grain, and also hay and straw chopped fine as food for cattle; hence, figuratively, the refuse or worthless part of anything. The colloquial use of the word, to chaff, in the sense of to banter or to make fun of a person, may be derived from this figurative sense, or from "to chafe," meaning to vex or irritate.

CHAFFARINAS, or ZAITARINES, a group of islands belonging to Spain off the north coast of Morocco, near the Algerian frontier, 2-1/2 m. to the north of Cape del Agna. The largest of these isles, Del Congreso, is rocky and hilly. It has a watch-house on the coast nearest to Morocco. Isabella II., the central island, contains several batteries, barracks and a penal convict settlement. The Spanish government has undertaken the construction of breakwaters to unite this island with the neighbouring islet of El Rey, with a view to enclose a deep and already sheltered anchorage. This roadstead affords a safe refuge for many large vessels. The Chaffarinas, which are the _Tres Insulae_ of the Romans and the _Zafran_ of the Arabs, were occupied by Spain in 1848. The Spanish occupation anticipated by a few days a French expedition sent from Oran to annex the islands to Algeria. The population of the islands is under 1000.

CHAFFEE, ADNA ROMANZA (1842- ), American general, was born at Orwell, Ohio, on the 14th of April 1842. At the outbreak of the Civil War he entered the United States cavalry as a private, and he rose to commissioned rank in 1863, becoming brevet captain in 1865. He remained in the army after the war and took part with distinction in many Indian campaigns. His promotion was, however, slow, and he was at the age of fifty-six still a lieutenant-colonel of cavalry. But in 1898, at the outbreak of the Spanish-American War, he was made brigadier-general and soon afterwards major-general of volunteers. In the Cuban campaign he won particular distinction, and the victory of the Americans in the action of El Caney was in large measure due to his careful personal reconnaissances of the ground to be attacked and to the endurance of his own brigade. After reverting for a time to the rank of brigadier-general, he was made a major-general U.S.V. again in 1900 and was appointed to command the United States contingent in China. He took a brilliant and successful part in the advance on Peking and the relief of the Legations. In 1901 he became a major-general in the regular army, and in 1901-1902 commanded the Division of the Philippines. In 1902-1903 he commanded the Department of the East, and from 1904 to 1906 was chief of the general staff of the army. In 1904 he received the rank of lieutenant-general in the United States army, being the first enlisted man of the regular army to attain this, the highest rank in the service. He was retired at his own request on the 1st of February 1906, after more than forty years' service.

CHAFFINCH (_Fringilla coelebs_), the common English name of a bird belonging to the family _Fringillidae_ (see FINCH), and distinguished, in the male sex, by the deep greyish blue of its crown feathers, the yellowish green of its rump, the white of the wing coverts, so disposed as to form two conspicuous bars, and the reddish brown passing into vinous red of the throat and breast. The female is drab, but shows the same white markings as the male, and the young males resemble the females until after the first autumn moult, when they gradually assume the plumage of their sex. The chaffinch breeds early in the season, and its song may often be heard in February. Its nest, which is a model of neatness and symmetry, it builds on trees and bushes, preferring such as are overgrown with moss and lichens. It is chiefly composed of moss and wool, lined internally with grass, wool, feathers, and whatever soft material the locality affords. The outside consists of moss and lichens, and according to Selby, "is always accordant with the particular colour of its situation." When built in the neighbourhood of towns the nest is somewhat slovenly and untidy, being often composed of bits of dirty straw, pieces of paper and blackened moss; in one instance, near Glasgow, the author of the _Birds of the West of Scotland_ found several postage-stamps thus employed. It lays four or five eggs of a pale purplish buff, streaked and spotted with purplish red. In spring the chaffinch is destructive to early flowers, and to young radishes and turnips just as they appear above the surface; in summer, however, it feeds principally on insects and their larvae, while in autumn and winter its food consists of grain and other seeds. On the continent of Europe the chaffinch is a favourite song-bird, especially in Germany, where great attention is paid to its training.

CHAFING-DISH (from the O. Fr. _chaufer_, to make warm), a kind of portable grate heated with charcoal, and used for cooking or keeping food warm. In a light form, and heated over a spirit lamp, it is also used for cooking various dainty dishes at table. The employment of the chafing-dish for the latter purpose has been largely restored in modern cookery.

CHAGOS, a group of atolls in the Indian Ocean, belonging to Britain, disposed in circular form round the Chagos bank, in 4 deg. 44' to 7 deg. 39' S., and 70 deg. 55' to 72 deg. 52' E. The atolls on the south and east side of the bank, which has a circumference of about 270 m., have disappeared through subsidence; a few--Egmont, Danger, Eagle, and Three Brothers--still remain on the east side, but most of the population (about 700) is centred on Diego Garcia, which lies on the south-east side, and is nearly 13 m. long by 6 m. wide. The lagoon, which is enclosed by two coral barriers and accessible to the largest vessels on the north side, forms one of the finest natural harbours in the world. The group, which has a total land area of 76 sq. m., is dependent for administrative purposes on Mauritius, and is regularly visited by vessels from that colony. The only product is cocoa-nut oil, of which about 106,000 gallons are annually exported. The French occupied the islands in 1791 from Mauritius, and the oil industry (from which the group is sometimes called the Oil Islands) came into the hands of French Creoles.