F, Foot.
L, Edge of the mantle not removed in the front part of the specimen.
s.o., Oesophagus.
af, Anus.
gg, Genital duct.
go, External opening of the same.
eg, Stem of the nephridium leading to no, its external aperture.
nk, Reflected portion of the nephridial stem.
ng, Fine caeca of the nephridium, which are seen ramifying
transversely over the whole inner surface of the pedal muscular
mass.]
_Nervous System._--There are no well-marked specialized ganglia in the
central nervous system, nerve-cells being distributed uniformly along
the cords. There are two pairs of longitudinal cords, a pedal pair
situated ventrally and united beneath the intestine by numerous
commissures, and a pallial pair situated laterally and continuous with
one another above the rectum (fig. 7). The four cords are all
connected anteriorly with the cerebral commissure which lies above the
buccal mass anteriorly. From the points where the cords meet the
cerebral commissure, arise on each an anterior labial commissure and a
stomatogastric commissure. The letter bears two ganglion swellings,
the buccal ganglia. The labial commissure gives off a subradular
commissure which also bears two ganglia, these being in close relation
to a special sense-organ called the subradular organ, an epithelial
projection with nerve-endings, lying in front of the radula and
probably gustatory in function. One osphradium or branchial olfactory
organ is usually present on each side, on either side of the anus on
the inner wall of the mantle, near the base of the last gill. In
_Lepidopleuridae_ an osphradium occurs at the base of each gill. The
sense organs of the shell-valves have already been described.
_Development._--The eggs may be laid separately invested by a
chitinous envelope, or as in _Ischnochiton magdalenensis_ they may
form strings containing nearly 200,000 eggs, or the ova may be
retained in the pallial groove and undergo development there, as in
_Chiton polii_ and _Hemiarthrum setulosum_. One species
_Callistochiton viviparus_ is viviparous and its ova develop without a
larval stage in the maternal oviduct. Segmentation is total and at
first regular, and is followed by invagination, the blastopore passing
to the position of the future mouth. By the development of a ciliated
ring just in front of the mouth the embryo becomes a trochosphere. In
the centre of the praeoral lobe is a tuft of cilia. Just behind the
ciliated ring is a pair of larval eyes which disappear in the adult;
these correspond to the cephalic eyes of Lamellibranchs. An ectodemic
invagination forms a large mucous gland on the foot, which is more or
less atrophied in adult life. The gonads originate by proliferation of
the anterior wall of the pericardium. The shell-valves arise as
transverse thickenings of the dorsal cuticle behind the ciliated ring,
the tegmentum being the first part formed.
_Classification_.
[Illustration: After Hubrecht, loc. cit.
FIG. 7.--Diagrams of the nervous system of Amphineura.
A, Proneomenia.
B, Neomenia.
C, Chaetoderma.
D, Chiton.
c, Cerebral ganglia.
s, Sublingual ganglia.
v, Pedal (ventral) nerve-cord.
l, Visceral (lateral) nerve-cord.
pc. Post-anal junction of the visceral nerve-cords.]
[Illustration: From Gegenbaur, _Elements of Comp. Anatomy._
FIG. 8.--Anterior part of the nervous system of _Chiton cinereus_,
in more detail.
B, Buccal ganglia (concerned with the odontophore).
C, Cerebral nerve-mass.
P, Pedal ganglion and commencement of pedal nerve-cord.
pl, Visceral nerve-cord. The sublingual ganglia are not lettered.]
Suborder I. EOPLACOPHORA, Pilsbry.--Tegmentum coextensive with
articulamentum, or the latter projecting in smooth unslit plates.
Fam. 1. _Lepidopleuridae._--Terminal margins of end valves never
elevated; form oval or oblong. _Lepidopleurus cancellatus_, Sow.
North Atlantic and Mediterranean; various abyssal species. _Hanleya
hanleyi_, Bean, north Atlantic. _Hemiarthrum Microplax_. The extinct
_Gryptochitonidae_, Pilsbry, with other Palaeozoic genera, narrow
and elongated in form with terminal margins of end valves elevated,
belong to this group.
Suborder II. MESOPLACOPHORA, Pilsbry.--Insertion plates well developed
and slit.
Fam. 2. _Ischnochitonidae._--All the valves with slits, and the
inner layer well covered by the outer.
Subfam. 1. _Ischnochitoninae._--No shell-eyes: sutural laminae
separated; slits in the valves 1-7 do not correspond with the ribs
of the tegmentum. _Ischnochiton, Trachydermon, Chaetopleura,
Stenoplax, Stenoradsia_.
Subfam. 2. _Callochitoninae._ With shell-eyes and united sutural
laminae. _Callochiton laevis_, North Atlantic and Mediterranean.
Subfam. 3. _Callistoplacinae._ No shell-eyes, slits in the valves
1-7 corresponding with the ribs of the tegmentum. _Callistochiton_
(viviparous). _Nuttalochiton._
Fam. 3. _Mopaliidae._ Each intermediate valve with a single slit;
girdle hairy. _Mopalia, Placiphorella, Plaxiphora, Placophoropsis._
Fam. 4. _Acanthochitonidae._ Valves immersed in the girdle, with
small tegmentum. _Acanthochiton_ (A_. fascicularis_, North Atlantic
and Mediterranean). _Spongiochiton, Katharina, Amicula, Cryptochiton_
(_C. stelleri_, arctic).
Fam. 5. _Cryptoplacidae._ Vermiform, with thick girdle and small
valves; insertion and sutural plates strongly drawn forward, sharp
and smooth. _Cryptoplax, Choneplax._
Suborder III. TELEOPLACOPHORA, Pilsbry.--All the valves, or at least
the seven anterior, with insertion plates cut into teeth by slits.
Fam. 6. _Chitonidae._ Characters of the suborder.
Subfam. 1. _Chitoninae._ No extra-pigmental eyes; insertion plates
with pectinations between the fissures. _Chiton, Eudoxochiton,
Trachyodon, Radsia._
Subfam. 2. _Toniciinae._ Extra-pigmental shell-eyes. _Tonicia,
Acanthopleura, Enoplochiton, Onithochiton, Schizochiton, Lorica,
Loricella, Liolophura._
Order 2.--APLACOPHORA, von Jhering.
_Chaetoderma_ was first described by S. Loven, in 1841, and was for a
long time believed to be a Gephyrean worm. _Neomenia_, mentioned first
by Michael Sars in 1868 under the name _Solenopus_, was afterwards
included among the Opisthobranchs by J. Koren and D.C. Danielssen. C.
Gegenbaur placed the two genera in a division of Vermes which he called
Solenogastres.
The chief points in which the Aplacophora differ from the Polyplacophora
are: (1) they are worm-like in shape; (2) there is no distinct foot, and
the mantle bears no shell-valves, but only numerous calcareous spicules;
(3) the digestive tube is straight.
_Neomenia_ and its allies are marine animals living at depths of 15 to
800 fathoms on soft muddy ground; they are found crawling on corals and
hydrozoa, on which they feed. The British genera are: _Neomenia,
Rhopalomenia_ and _Myzomenia_. They have been taken in nearly all seas
except the South Atlantic and S.E. and N.W. Pacific. About forty species
are known. _Chaetoderma_, of which nine species have been described, has
similar habits and distribution, but feeds chiefly on Protozoa. The
order Aplacophora is divided into two suborders.
Suborder I. NEOMENIOMORPHA.--Aplacophora with a distinct longitudinal
ventral groove; bisexual with paired genital glands and no distinct
liver. The whole of the skin except the ventral groove corresponds to
the mantle of _Chiton_. The cuticle, in some species very thick,
contains numerous spicules which are long, hollow and calcified; they
are secreted by epithelial papillae. In some species there are also
sensory papillae comparable to the aesthetes of Chitons. A small
longitudinal projection in the ventral groove represents the foot.
Into the groove open mucous glands, a large one anteriorly and another
opening into a posteriorly cloacal, branchial cavity.
[Illustration: FIG. 9.--_Neomenia carinata_, Tullberg (after Tullberg).
A, Lateral view.
B, Ventral view.
C, Dorsal view.
D, Ventral view of a more extended specimen.
a, Anterior.
b, Posterior extremity.
c, Furrow, in which the narrow foot is concealed.]
_Branchiae._--In _Neomeniidae_ and most of the _Parameniidae_ there is
a circlet of gills on the inner walls of the cloacal chamber. These
gills are simple folds or laminae of the body wall. In other species
they are absent.
_Intestine._---The mouth opens into a muscular pharynx lined by a
thick cuticle. Into the pharyngeal cavity open salivary glands and
radular sac. The former are paired and ventral, and open on a
subradular prominence. In some species there is a second dorsal pair.
_Neomenia_ and other genera have no salivary glands.
The radula when present comprises several transverse rows of teeth,
and each transverse row may have several teeth (polystichous), two
teeth (distichous), or one tooth (monostichous). It is a curious fact
that in the original type _Neomenia_ the radula is entirely absent, as
it likewise is in several genera of _Proneomeniidae_. The oesophagus
is short and leads into a long, straight stomach, provided with
numerous symmetrical lateral caeca. The stomach opens into a short
straight rectum which opens into the branchial chamber.
_Coelom, Gonads and Excretory Organs._--The coelom differs from that
of the Chitons in the fact that the cavities of the genital organs are
continuous with it, and in the fact that there is only one pair of
coelomoducts resembling the renal organs of Chitons, but serving also
as genital ducts. The gonads are paired and hermaphrodite, they form a
pair of anterior prolongations of the pericardium, extending nearly to
the anterior end of the body. Ova are developed on the median,
spermatozoa on the outer wall of each genital tube. The pericardium is
ciliated internally on its dorsal and lateral walls. The urino-genital
tubes arise from the posterior angles of the pericardium, pass first
forwards, then backwards, and unite to open by a common opening into
the cloaca below the anus except in _Strophomenia_, where the openings
are separate. Usually each tube is provided with caecal appendages on
its proximal portion, and these serve as vesiculae seminales, while
the distal portion is enlarged and glandular and secretes the
egg-shell.
_Heart and Vascular System._--There is a heart in the pericardium
consisting of a median ventricle attached, except in _Neomenia_, to
the dorsal wall of the pericardium, and in _Neomenia_ a pair of
auricular ducts returning blood from the gills to the ventricle. The
aorta is not independent as in Chitons, but is a sinus like the other
channels of the circulation. A single median ventral sinus passes
backwards to the gills or cloaca. The blood is coloured red by
haemoglobin in blood corpuscles.
_Nervous System._--Ganglionic enlargements are more conspicuous than
in the Chitons. In front of the buccal mass is a median cerebral
ganglion. From this pass off two pairs of cords, the pleural and
pedal, in _Proneomenia_ separate from their origin, in _Neomenia_
united at first and diverging at a pleural ganglion. The pedal cords
anteriorly form a pair of pedal ganglia united by a thick commissure.
The supra-rectal commissure may be present and bear an ovoid ganglion;
or may be wanting. With regard to sense organs the epithelial papillae
of the mantle have been mentioned. There is also in some genera a
median retractile sensory papilla on the dorsal posterior surface
above the rectum, not covered by the cuticle.
_Development_ has only been described in _Myzomenia banyulensis_, by
G. Pruvot. It closely resembles in the early stages that of Chitons.
The external surface of the trochosphere is formed of a number of
ciliated test-cells. The ectoderm behind the ciliated ring develops
spicules, and the post-oral region of the larva elongates. Later the
ciliated ring or velum disappears and seven imbricated calcareous
plates, made up of flattened spicules, are formed on the dorsal
surface. This appears to indicate that the Neomeniomorpha are
descended from _Chiton_-like ancestors, and that they have lost their
shell valves.
_Classification of the_ NEOMENIOMORPHA.--Fam. 1. _Lepidomeniidae._
Slender, tapering behind, with subventral cloacal orifice; thin
cuticle without papillae; flattened spicules; no gills.
_Lepidomenia, Ismenia, Ichthyodes, Stylomenia, Dondersia,
Nematomenia, Myzomenia, M. banyulensis_, Mediterranean and Plymouth.
Fam. 2. _Neomeniidae._ Short, truncate in front and behind; cloacal
orifice transverse; gills present; rather thin cuticle; no radula.
_Neomenia_ (_N. carinata_, N. Atlantic and N. and N.W. Scotland),
_Hemimenia_.
Fam. 3. _Proneomeniidae._ Elongated, cylindrical, rounded at both
ends; thick cuticle with acicular spicules; radula polystichous or
wanting. _Proneomenia, Amphimenia, Echinomenia, Rhopalomenia_ (_R.
aglaopheniae_, Mediterranean and Plymouth), _Notomenia, Pruvotia,
Strophomenia_.
Fam. 4. _Parameniidae._ Short and truncated in front; thick cuticle,
often without papillae; gills and radula present. _Paramenia,
Macellomenia, Pararhopalia, Dinomenia, Cyclomenia, Proparamenia,
Uncimenia, Kruppomenia._
Suborder II. CHAETODERMOMORPHA.--Aplacophora without distinct ventral
groove, with single median unisexual gonad, with differentiated
hepatic sac, and with cloacal chamber furnished with two bipectinate
gills. There are only two genera in this suborder: _Chaetoderma_, and
_Limifossor_ from Alaska. The characters therefore are very uniform.
The body is worm-like and cylindrical, the posterior half a little
thicker than the anterior; the posterior extremity forms the enlarged
funnel-like branchial or cloacal chamber. The anterior extremity is
also somewhat enlarged. The whole surface is uniformly covered with
short compressed calcareous spicula embedded in the cuticle.
[Illustration: FIG. 10.--_Chaetoderma nitidulum_, Loven (after Graff).
The cephalic enlargement is to the left, the anal chamber (reduced
pallial chamber, containing the concealed pair of ctenidia) to the
right.]
_Branchiae._--The single pair of branchiae are placed symmetrically
right and left of the anus, and each has the structure of a ctenidium
bearing a row of lamellae on each side as in the Polyplacophora.
_Intestine._--The mouth is anterior, terminal and crescentic, and
beneath it is a rounded ventral shield. On the floor of the pharynx or
buccal mass is a rudimentary radula, which in many species consists of
a single large tooth, bearing two small teeth or a row of teeth. In
other species the radula is more of the usual type consisting of
several transverse rows of two or three teeth each. Two pairs of
salivary glands open into the buccal cavity. The digestive tube is
straight and simple, wider in its anterior part, into which opens the
duct of the hepatic caecum (fig. 4, B). The latter extends backwards
on the ventral side of the intestine.
_Coelom, Gonads and Excretory Organs._--These are closely similar in
their relations to those of the Neomeniomorpha. The chief difference
is that the gonad or generative portion of the coelom is single and
median, opening into the pericardium by a single posterior aperture.
The excretory organs or coelomoducts arise from the posterior corners
of the pericardium, run forwards and then backwards to open by
separate apertures lateral to the gills (fig. 5, A). There are no
accessory generative organs.
_The heart and vascular system_ are similar to those of the
Neomeniomorpha, the only important differences being that the
ventricle is nearly free in the pericardial cavity, and that the
latter is traversed by the retractor muscles of the gills.
_Nervous System._--There are two closely connected cerebral ganglia,
from which arise the usual two pairs of nerve cords. Pallial and pedal
on each side are closer together than in the other groups, and
posteriorly they unite into a supra-rectal cord provided with a median
ganglionic enlargement (fig. 7, C). A small stomatogastric commissure
bearing two small ganglia arises from the cerebral ganglia and
surrounds the oesophagus.
The development is at present entirely unknown.
_General Remarks on the Amphineura._
The most important theoretical question concerning the Amphineura is how
far do they represent the original condition of the ancestral mollusc?
That is to say, we have to inquire which of their structural features is
primitive and which modified. Their bilateral symmetry is obviously to
be regarded as primitive, and the nervous system shows an original
condition from which that of the asymmetrical twisted Gastropods can be
derived. But in many other features both external and internal the three
principal divisions differ so much from one another that we have to
consider in the case of each organ-system which condition is the more
primitive. According to Paul Pelseneer the Polyplacophora are the most
archaic, the Aplacophora being specialized in (1) the great reduction of
the foot, (2) the disappearance of the shell (_Cryploplax_ among the
Polyplacophora showing both reductions in progress), (3) the
disappearance of the radula. But it is a widely recognized principle of
morphology that a much modified animal is by no means modified to the
same degree in all its organs. A form which is primitive on the whole
may show a more advanced stage of evolution in some particular system of
organs than another animal which is on the whole more highly developed
and specialized. Thus the independent metamerism of certain organs in
the Chitons is not primitive but acquired within the group: e.g. the
shell valves and the ctenidia. And although embryology seems to prove
that the Neomeniomorphs are derived from forms with a series of
shell-valves, nevertheless it seems probable that the calcareous
spicules which alone are present in adult Aplacophora preceded the solid
shell in evolution.
It is held by some morphologists that the mollusc body is unsegmented,
and therefore is to be compared to a single segment of a Chaetopod or
Arthropod. In this case there should be only one pair of coelomoducts in
the adult, the pair of true nephridia which should also occur being
represented by the larval nephridia. There should also be only a single
coelom, or a pair of lateral coelomic cavities. On this view then the
Aplacophora are more primitive than the Polyplacophora in the relations
of coelom, gonad and coelomoducts; and the genital ducts of the Chitons
have arisen either by metameric repetition within the group, or by the
gradual loss of an original connexion between the generative sac and the
renal tube, as in Lamellibranchs and Gastropods, the generative sac
acquiring a separate duct and opening to the exterior on each side.
LITERATURE.--A. Sedgwick, "On certain Points in the Anatomy of
Chiton," _Proc. R. Soc. Lond._ xxxiii., 1881; J. Blumrich, "Das
Integument der Chitonen," _Zeitsch. f. wiss. Zool._ lii., 1891; A.C.
Haddon, "Report on the Polyplacophora," _Challenger Reports. Zool._
pt. xliii., 1886; H.N. Moseley, "On the presence of Eyes in the Shells
of certain Chitonidae, and on the structure of these Organs," _Quart.
Journ. Mic. Sci._ new ser. xxv., 1885; A.A.W. Hubrecht, "Proneomenia
Sluiteri," _Nied. Arch. f. Zool._ Suppl. 1., 1881; A. Kowalewsky and
A.F. Marion, "Contr. a l'histoire des Solenogastres ou Aplacophores,"
_Ann. Mus. Marseille, Zool._ iii., 1887; A. Kowalewsky, "Sur le genre
Chaetoderma," _Arch. de zool. exper_. (3) ix., 1901; P. Pelseneer,
"Mollusca," _Treatise on Zoology_, edited by E. Ray Lankester, pt. v.,
1906; E. Ray Lankester, "Mollusca," in the 9th ed. of this
Encyclopaedia, to which this article is much indebted. (J. T. C.)