Acta Zoologica (Stockholm) 89: 137–148 ( April 2008)
doi: 10.1111/j.1463-6395.2007.00301.x
On the fine structure of the creeping larva of Loxosomella
murmanica: additional evidence for a clade of Kamptozoa
(Entoprocta) and Mollusca
Blackwell Publishing Ltd
Gerhard Haszprunar1 and Andreas Wanninger2
Abstract
1
Zoologische Staatssammlung München,
Münchhausenstrasse 21, D-81247
München, Germany; 2Department of Cell
Biology and Comparative Zoology, Institute
of Biology, University of Copenhagen,
DK-2100 Copenhagen, Denmark
Keywords:
Entoprocta, Kamptozoa, larva, Mollusca,
phylogeny, ultrastructure
Accepted for publication:
26 June 2007
Haszprunar, G. and Wanninger, A. 2008. On the fine structure of the creeping
larva of Loxosomella murmanica: additional evidence for a clade of Kamptozoa
(Entoprocta) and Mollusca. – Acta Zoologica (Stockholm) 89: 137–148
The creeping larva of the kamptozoan (entoproct) Loxosomella murmanica was
investigated using transmission electron microscopy. The late larva exhibits a
prominent apical organ connected to the ‘cerebral’ commissure of large
cerebral ganglia, which supply the paired frontal organ. From the cerebral
ganglia two paired nerve cords project backwards, closely resembling the
tetraneuralian pattern of basal molluscs. In addition, a neural ring supplying
the prototroch is present. The epidermis is composed of myoepithelial cells.
Dorsally its cuticle is covered by granules of unknown composition. The
prototroch consists of two ciliary rings; a downstream collecting system is not
present. Although there is a one-way gut with a lumen throughout, the larva
obviously does not feed. A single pair of protonephridia is present. The foot
sole shares distinct similarities with basic molluscs, particularly with those of
the aplacophoran Solenogastres:The anterior part shows a huge, subepidermal
pedal gland and several bundles of cirri consisting of compound cilia. The
posterior part is ciliated with intraepithelial mucous cells interspersed.
The dorsoventral muscle fibres show the mollusc-like ventral intercrossing. The
present results and previous findings, in particular the chitinous, non-moulted
cuticle, the sinus circulatory system, and a number of neural features shared
by Kamptozoa and Mollusca, provide substantial evidence for a direct sistergroup relationship between these phyla. In addition, the basal position of the
Solenogastres (Neomeniomorpha) within the Mollusca is corroborated.
Gerhard Haszprunar, Zoologische Staatssammlung München, Münchhausenstrasse 21, D-81247 München, Germany. E-mail: haszi@zsm.mwn.de
Introduction
Whereas the systematic position of Kamptozoa (= Entoprocta)
is fairly settled among the spiralian Lophotrochozoa by
phenotypic and genotypic characters, clear affinities of
this phylum are still a matter of debate (see the recent review
by Nielsen 2002a). Contrary to the statement of Nielsen
(2002a: p. 687) that ‘no entoproct character indicates a closer
relationship with any specific spiralian phylum’, Bartolomaeus (1993a,b) and Haszprunar (1996) have both argued in
© 2007 The Authors
Journal compilation © 2007 The Royal Swedish Academy of Sciences
favour of a direct sister-group relationship with the Mollusca.
These assumptions were based on several similarities
between Kamptozoa and basal Mollusca: (1) a distinct
division of the body into a dorsal area covered by a cuticle and
a ventral area that is specialized as a ciliated creeping sole
with anterior cirri is shared by basal Mollusca (particularly
the aplacophoran Solenogastres) and creeping entoproct
larvae; (2) an epidermal, chitinous cuticle, which is not
moulted, in contrast to the ecdysozoan Nemathelminthes
and Arthropoda; (3) a sinus circulatory system, where the
Fine structure of Loxosomella larva (Entoprocta) • Haszprunar and Wanninger
blood flows in distinct spaces lacking any epithelial border,
combined with a pumping organ (Emschermann 1969).
Based on a cladistic analysis of morphological characters of
the spiralian phyla, Haszprunar (1996: p. 22) concluded that
‘the assumption of the proposed sister-group relationship
[of Mollusca and Kamptozoa] is now more probable than
previously thought,’ but ‘that data on ... the kamptozoan
larva are still very poor’.
Indeed, the aberrant morphology and biology of the
sessile, filter-feeding kamptozoan postlarval stages makes it
improbable that distinct morphological similarities with any
other spiralian phylum will be found. It is much more likely
that the trochophore-like larvae of the Kamptozoa do exhibit
such similarities. However, up to now only two studies dealt
with the fine-structure of kamptozoan larvae: Woollacott
and Eakin (1973) described the fine structure of the cerebral
eyes of the creeping larva of Loxosomella cf. harmeri, and
Sensenbaugh (1986, 1987) investigated the holoplanktonic
larva of Loxosoma pectinaricola using transmission electron
microscopy (TEM). In addition, Nielsen provided scanning
electron micrographs (SEM) of the larval Loxosoma pectinaricola
and of an unidentified creeping loxosomatid larva from the
Bahamian plankton (Nielsen 2002a,b). Based on widespread
and thorough studies on the kamptozoan life cycle, Nielsen
(1971) concluded that the creeping type with direct metamorphosis is probably the most primitive larval type among
the Kamptozoa. Therefore, we concentrated our studies on
this larval type to look for peculiarities with phylogenetic
significance. Most recently, Wanninger et al. (2007) pointed
out the characteristic nervous system of this larval type
revealed by immunocytochemistry and confocal laser
scanning microscopy (CLSM). Here we describe the fine
structure of the kamptozoan creeping larva and discuss the
molluscan affinities of the Kamptozoa.
Materials and Methods
Animals
Adult Loxosomella murmanica (Nilus 1909) live on the body
wall of the sipunculan Phascolion strombus, which inhabits
empty shells of the gastropod Turritella (cf. Nielsen 1971).
Turritella shells inhabited by Phascolion were dredged from
30 m depth off the Kristineberg Marine Research Station,
Sweden, transported to the laboratory, and maintained in
large plastic containers in natural seawater. Water was
changed twice per day; the seawater was filtered through a
70-µm mesh and checked using a stereomicroscope for
larvae that had been released by the mother animal.
Fixation and TEM
All individuals were relaxed in a solution of 3.5% MgCl2
before fixation, which was carried out using a solution of 4%
glutaraldehyde in 0.2 sodium cacodylate buffer (SCB) for
Acta Zoologica (Stockholm) 89: 137–148 (April 2008)
2 h at room temperature. Subsequently, the samples were
washed three times for 15 min in SCB and stored at 4 °C.
Alternatively, larvae that were fixed in 4% paraformaldehyde
with 0.05% glutaraldehyde and stored in phosphate-buffered
saline with 0.1% NaN3 were used with the same results. In
the following, the specimens were post-fixed in buffered 1%
OsO4 for 1 h at room temperature, washed in deionized
water (three times for 15 min), dehydrated in an ethanol
series and embedded in low-viscosity resin (Spurr 1969).
Ultrathin sections with gold–silver interference colour (about
70 nm thick) were cut with a diamond knife and stained for
30 min with saturated uranyl acetate and for 5 min with lead
citrate after Reynolds (1963). The sections were examined
and photographed with a Philips CM 10 TEM.
Results
External morphology
The larvae investigated closely resemble the general descriptions given by Jägersten (1964) for Loxosomella, and by
Franzén (1970) and Nielsen (1967, 1971) for the same
species. Peculiarities of this creeping larva are a prominent
foot sole, the lack of pigmented eye spots, the prominent pair
of ciliary frontal organs, and the covering of the dorsal side
by granules of unknown composition (Fig. 1).
Compared with other trochophore larvae, the axes in
Loxosomella are somewhat different in that the pretrochal
episphere is extended and covers the whole dorsal body of
the larva.
The larva is of the ciliary-gliding type with a prominent
foot sole that bears some cirri at the frontal end. Most anteriorly there are the paired, frontal organs (with extended cilia
in live animals, but contracted in the preserved specimens),
each of which has a prominent ciliary tuft, but they lack
the pigmented eye spots found in the larvae of many other
loxosomatid species. The dorsal area (episphere) is covered
by a thin cuticle and shows fine granules. The prototroch
forms a ring around the whole ventral side of the body and
can be contracted so that the foot is enclosed in a cavity.
Episphere
The epidermal cells of the episphere lack cilia and are
covered by a thin microvillous cuticle (Rieger 1984). Over a
distance of about 200 nm the glycocalyces of the microvilli
form a dense layer, the microvilli themselves are slightly
longer than this distance (Fig. 2A). This type of cuticle also
covers the prototroch region, where the cilia penetrate the
cuticle (Fig. 2B). At the episphere there are additional granulelike elements outside the cuticle, which strongly react with
the electron beam in the TEM showing dynamic interference
patterns (Fig. 2C). We could not detect any evidence that
these particles are produced by the animal. Most epidermal
cells of the episphere are myoepithelial cells, where the
© 2007 The Authors
Journal compilation © 2007 The Royal Swedish Academy of Sciences
Acta Zoologica (Stockholm) 89: 137–148 ( April 2008)
Haszprunar and Wanninger • Fine structure of Loxosomella larva (Entoprocta)
Nervous system
The anatomy of the serotonergic portion of the nervous
system has been recently described in detail by Wanninger
et al. (2007). The TEM sections confirm and supplement the
theory that the general pattern of the nervous system
matches the results provided by antiserotonin staining: we
found a paired cerebral ganglion supplying the frontal
organs, and we confirm the subepidermal, tetraneuran
condition of the longitudinal neural cords (Fig. 3). One pair
of prominent nerves (Fig. 4A) runs above the foot sole, a
second pair, which is less prominent (Fig. 4B), is situated
laterally at half the level of the episphere. Pericarya of nerve
cells are rarely found, although they are quite large. In addition, there is a very large apical ganglion with – contrary to
the apical organ itself – almost no serotonergic elements, and
the likewise subepidermal, circumprototrochal nerve ring is
prominently developed (Fig. 3).
There are no further main nerves or ganglia after those
revealed by the antiserotonin staining.
Apical organ
The apical organ consists of many, very narrow, polyciliary
cells without any cuticle formation, but with a dense microvillous border and few supporting cells interspersed. The
cilia of the apical organ show the regular microtubular
pattern and have short roots (Fig. 4C). The basal neural
processes of the ciliary cells run directly downwards into the
underlying, very large apical ganglion.
Frontal organ
Fig. 1—The creeping larva of Loxosomella murmanica. Scale bars
represent 50 µm. —A. SEM image in ventral aspect showing the
ciliated gliding sole of the foot (ft), the ciliated area of the paired
frontal organ (f1, f2), as well as the prototroch (pt). —B. Drawing
of a live specimen in lateral left view. Note the cirri (cr) situated at
the anterior region of the foot and the apical organ (ao), which is
located opposite the foot and so is shifted by 90° relative to the
anterior–posterior axis of the larva. After Nielsen (1971),
reproduced with kind permission of the author.
The paired frontal organ lies anteriorly between the apical
organ and the mouth opening and is extended in the living
larva (Fig. 1). In the preserved specimens it consists of two
deep invaginations that are filled by densely arranged cilia
from several polyciliary cells (Fig. 4D). The cilia show the
regular pattern of microtubuli and long roots.
The cerebral ganglia are situated immediately behind the
cavities of the frontal organs. The short cerebral commissure
fuses mid-dorsally with the ventral part of the apical ganglion
(Fig. 5A).
Prototroch
myofilaments are obliquely striated and are mainly situated
in the distal part of the cells interconnecting the zonula adherens
of the belt desmosomes (Fig. 2D).
Large, solitary mucous cells occur occasionally in the
epidermis of the episphere (Fig. 2E). The nucleus rests on
the base and is often highly homogeneous, the cytoplasm
shows several large vacuoles; a distinct opening was never
detected.
© 2007 The Authors
Journal compilation © 2007 The Royal Swedish Academy of Sciences
The circular prototroch surrounds the whole body of
the larva and forms a distinct fold. The prototroch itself
consists of two cell rows separated from each other by a cell
row of obliquely striated myoepithelial cells (Fig. 5B),
whereas a ciliated food rim is lacking. The dorsal ciliary
ring (i.e. the ‘outer prototroch’ of Jägersten 1964) is formed
by cells with very many (up to 40 have been counted),
long, accurately arranged cilia with very long roots reaching
to the basis of these cells (Fig. 5B,C). The myoepithelial
Fine structure of Loxosomella larva (Entoprocta) • Haszprunar and Wanninger
Acta Zoologica (Stockholm) 89: 137–148 (April 2008)
Fig. 2—Loxosomella murmanica, TEM. Orientation is with the dorsal side facing upwards, scale bars represent 1 µm. —A. Cross-section
of the ‘pallial region’ between prototroch and foot showing the microvillous border (mv) with distinct glycocalyx layer (arrows).
—B. Cross-section of the outer prototroch showing distally situated mitochondria (asterisks), accurately arranged cilia with long roots
(arrowheads), microvillous border (mv), and glycocalyx layer (arrow). —C. Epidermis of the episphere showing microvillous border (mv) with
glycocalyx (obliquely sectioned) and the enigmatic granules (gr). Below the epidermis, muscle fibres (mu) are visible. —D. Tangential section
of the epidermal myoepithelial cells in the prototroch region with cilia (ci). Note that the myofilaments (mf ) are situated distally of the cell.
—E. Solitary mucous cells of the episphere. Note the nucleus (nu) with homogeneous chromatin, the large, bright vacuoles (va), and the
sparse regular cytoplasm (cy).
cells form a ring, which probably corresponds with the
‘prototroch constrictor’ described by Nielsen (1971:
Fig. 31) for Loxosomella pectinaria, although the trochus
conditions differ in that species. The ventral ring (‘middle
metatroch’ of Jägersten 1964) is much smaller than the
outer ring, the respective cells bear fewer and much
shorter cilia. According to the orientation of the ciliary
microtubules, the beating direction of the cilia (always
perpendicular to the line of the central tubuli towards the
cleft between tubuli pairs 4 and 5) of both rings is from dorsal
to ventral.
Foot
The cilia of the most anterior portion of the foot typically
form cirri (Mariscal 1965; Nielsen 1967, 1971; Franzén
1970). The TEM images show, however, that a special
connection between the cilia does not exist. Between the
polyciliary cells of this region there are the openings of the
cells of the pedal glands (Fig. 5D). These symmetrically
arranged glands reach deep into the head region of the larva
and occupy a large volume dorsal to the gut (Fig. 6A). The
cytoplasm of these subepidermal gland cells show numerous,
© 2007 The Authors
Journal compilation © 2007 The Royal Swedish Academy of Sciences
Acta Zoologica (Stockholm) 89: 137–148 ( April 2008)
Haszprunar and Wanninger • Fine structure of Loxosomella larva (Entoprocta)
because of the positional shift of the prototroch by 90°
relative to other trochozoan larvae, the axis of the gut
corresponds to the longitudinal axis of the prototroch and
does not cross the latter as in the typical trochophore-type
larva. The mouth opening is placed behind the anterior fold
of the prototroch. The most anterior part of the gut makes a
short loop forwards and upwards and then runs straight
backwards. The hindgut opens dorsally near the posterior
end of the foot (Fig. 6D). The lateral cells of the oral opening
and of the most anterior part of the gut have a microvillous
border and cilia. The anterior portion of the gut opens quite
abruptly into the midgut (Fig. 7A). The epithelium of the
midgut shows a dense microvillous border and small yolk
vesicles but no cilia, the lumen is extremely narrow. The cilia
of the anterior part reach backwards for some distance, later
only microvilli are visible in cross-sections. The cells of the
posterior alimentary channel and the anal opening again
have cilia. There was no trace of food or nourishment
throughout the whole gut, the cells of which still show some
small yolk vesicles.
Protonephridia
Fig. 3—Loxosomella murmanica, TEM. Orientation is with the dorsal
side facing upwards. —A. Cross-section of the larval body at half
length showing the general organization (cuticularized episphere,
ciliated prototroch, pallial rim lateral to the ciliated foot sole), in
particular the central gut and the position of the lateral and pedal
nerve cords (see inset figure): two asterisks indicate artificial cleft
caused by fixation above the gut, one asterisk indicate a hole in the
section caused by a sticky stone. Scale bar = 25 µm. —B. Same
photograph with marked gut (dark grey, central position), lateral
and pedal nerve cords (white; the right lateral cord is swollen
because of the presence of a nerve cell body with nucleus, see
Fig. 4B), and the subepidermal prototroch nerve (grey, lateral).
small, homogeneous vesicles (Fig. 6B), while their necks are
quite narrow.
More posteriorly, the foot sole becomes regularly ciliated,
mucous cells are rare in this region and always truly epithelially positioned. The microvillous border of this area again
shows distally a very delicate, but distinct glycocalyx layer. In
this area there are dorsoventral muscles, which are obliquely
striated and run from the dorsolateral region to the foot.
These muscles are obliquely striated and intercross just
above the foot sole’s epithelium (Fig. 6C).
Alimentary tract
The free larval stage investigated has a gut, the lumen of
which runs throughout the body. It is worthwhile noting that,
© 2007 The Authors
Journal compilation © 2007 The Royal Swedish Academy of Sciences
A single pair of laterally situated protonephridia could be
detected in the TEM sections. There is a single, multiciliated
terminal cell building up the ultrafiltration weir, its nucleus
is semicircular in shape and is thus visible twice in the
sections (Fig. 7B). The channel is formed by a string of single
cells, its lumen is extremely narrow and is entirely filled with
cilia (Fig. 7C). Close to the excretory opening the channel
bends towards the ventral side, and after this the lumen
is widened and the cells bear microvilli (Fig. 7D). The
excretory opening is provided with a sphincter muscle and is
placed in the lateral ‘pallial’ rim between the prototroch and
the foot.
Discussion
The kamptozoan gliding larva as a trochophore – molluscan
mosaic
The present results provide novel insights into the organization of the kamptozoan creeping larva. According to Nielsen
(1971), this larval type is more primitive for the phylum than
both the holopelagic type with a reduced foot sole and the
intermediate forms that have been described (Nielsen 1971).
The larva of L. murmanica exhibits a mosaic of (1) autapomorphic characters, which may additionally define the
phylum but need confirmation by studies on larvae of
further species, (2) typical features of a trochophore larva
being defined in the broad sense of Rouse (1999) as the
synapomorphic, lecithotrophic larva of Trochozoa, i.e.
Kamptozoa, Mollusca, Sipuncula and Annelida (possibly
also Nemertinea; cf. Maslakova et al. 2004), (3) characters
that are shared with the adult Mollusca, particularly with
Fine structure of Loxosomella larva (Entoprocta) • Haszprunar and Wanninger
Acta Zoologica (Stockholm) 89: 137–148 (April 2008)
Fig. 4 —Loxosomella murmanica, TEM.
Orientation is with the dorsal side facing
upwards, scale bars represent 1 µm in A–C
and 10 µm in D. —A. Detail of Fig. 3.
Cross-section of the pedal nerve above the
ciliated epithelium of the foot sole (cf )
lateral to the dorsoventral muscle fibre (mf ).
—B. Detail of Fig. 3. Cross-section of the
lateral nerve below the cuticularized
epidermis (ep) of the episphere showing a
(rare) neural nucleus (nu) with two portions
of neuropile (ne) adjacent to various muscle
fibres (mf ). —C. Cross-section of the apical
region showing many narrow sensory cells
(asterisks), bearing long cilia (ci) with short
roots (arrowheads). In living larvae the cilia
form a ciliary tuft. —D. Cross-section of the
most anterior part of the larva showing the
retracted frontal organs (fo) with dense
ciliation (ci). In the living larva the frontal
organs are extended (see Fig. 1).
Fig. 5—Loxosomella murmanica, TEM.
Orientation is with the dorsal side facing
upwards, scale bars represent 10 µm in A
and 1 µm in B–D. —A. Cross-section of the
larva just in front of the apical organ (ao)
showing the large, centrally placed apical
ganglion (ag) and more ventrally the
posterior portion of the paired cerebral
ganglion (cg). Dorsally right several
obliquely striated muscle fibres of the
dorsoventral muscles (dvm) are visible.
—B. Cross-section of the prototroch
showing the dorsally placed main trochus,
the cilia (ci) with very long roots marked
by arrowheads, the myoepithelial cell row
(me), and the much weaker ventral trochus
(vt). —C. Oblique section of the outer
prototroch showing accurately arranged
cilia (ci) with long roots (dark dots in the
cytoplasm) between many distally situated
mitochondria (asterisks). —D. Crosssection of the anterior end of the foot sole
showing the releasing necks and openings of
the pedal gland cells (pg) between the
polyciliary cells bearing the cirri (cr).
© 2007 The Authors
Journal compilation © 2007 The Royal Swedish Academy of Sciences
Acta Zoologica (Stockholm) 89: 137–148 ( April 2008)
Haszprunar and Wanninger • Fine structure of Loxosomella larva (Entoprocta)
Fig. 6—Loxosomella murmanica, TEM.
Orientation is with the dorsal side facing
upwards, scale bars represent 10 µm in A,
1 µm in B and C, and 2 µm in D.
—A. Cross-section posterior to the cerebral
ganglia. In the centre lies the foregut (fg),
the lumen of which is filled by cilia. Dorsally
placed are the pedal glands (pg), the cleft is
an artefact. Ventral side with ciliary foot sole
(fs), flanked by the releasing channels (rc) of
the pedal glands. Most laterally are the
polyciliary cells of the prototroch (pt).
—B. Cells of the pedal glands showing
nuclei (nu), adjacent dictyosomes (di),
smooth endoplasmic reticulum, and
numerous vesicles. —C. Cross-section of
the central foot sole with dense ciliation
(ci). Note the dorsoventral muscles (dvm),
which intercross ventrally (square).
—D. Cross-section of the posterior end of
the larva showing the prototroch (pt) and
the anal opening (a) slightly dorsal to the
rear of the foot sole (fs).
Fig. 7—Loxosomella murmanica, TEM.
Orientation is with the dorsal side facing
upwards, scale bars represent 1 µm.
—A. Cross-section of the point where the
foregut (fg) with many cilia enters the
midgut (mg) with much denser microvilli
and few cilia. —B. Oblique section through
the terminal end of the protonephridium
showing the cilium (ci) of the flame cell with
the ultrafiltration weir (arrows) with many
microvilli and (because of bending) a crosssection of the most terminal part of the
releasing channel (rc) filled with cilia. The
various sections all belong to the single
nucleus (nu) of the terminal cell.
Dorsoventral and longitudinal muscle fibres
(mf ) are situated nearby. —C. Cross-section
of the releasing channel (rc) of the
protonephridium filled with cilia adjacent to
the lateral nerve (ne). —D. Cross-section of
the protonephridial releasing channel (rc)
close to the excretory opening. Note the cilia
and microvilli in the lumen and the adjacent
sphincter muscle (m).
© 2007 The Authors
Journal compilation © 2007 The Royal Swedish Academy of Sciences
Fine structure of Loxosomella larva (Entoprocta) • Haszprunar and Wanninger
the Solenogastres (Neomeniomorpha), and (4) neuroarchitectural apomorphies shared with larval polyplacophorans (Friedrich et al. 2002; Voronezhskaya et al. 2002; the
larval nervous system of Solenogastres and Caudofoveata is
still insufficiently known). All character sets coexist beside
each other in this larva, which as a whole may be interpreted
as a trochophore–ancestor mosaic with some peculiarities.
The general morphology of the creeping larva is twofold:
on the one hand it is a typical trochophore larva in the sense
of Rouse (1999), showing a ciliated apical sensory organ, a
prominent prototroch arising from the 1d-lineage (Marcus
1939; recently reviewed by Nielsen 2005), and a pair of
protonephridia. On the other hand, it exhibits several
characters that are typical for adult Mollusca: the division of
the body into a cuticularized episphere and a microvillous–
ciliary hyposphere, the ventral foot sole with its anteriorly
placed cirri, and the anal opening, which is situated slightly
dorsally to the rear of the animal. Moreover, the larval
serotonergic nervous system shows a mosaic of larval and
adult molluscan characters including a complicated larval
apical organ comprising a set of central flask cells and several
peripheral cells, a preoral nerve loop, an oral nerve ring, and
true mollusc-like tetraneury (Wanninger et al. 2007).
The episphere again shows the mosaic character of the
larva. Certainly the clearest apomorphic character of the
creeping larva of L. murmanica is found in the myoepithelial
cells of the episphere and prototroch (see also Sensenbaugh
1987). Such a cell-type is not known from any other trochophore type, but from adult Kamptozoa (Emschermann
1982). Eeckhaut and Jangoux (1993) have described similar
conditions in the epidermis of adult Myzostomida (the finestructure of their trochophore larvae is unknown), a phylum
probably closely related to the Annelida. It is thus unlikely
that this character represents a synapomorphy between the
two taxa. Again apomorphic, and possibly restricted to this
species, are the enigmatic granules on the epispheral cuticle,
the function and origin of which are unknown.
The general type of microvillous cuticle with a distally
placed glycocalyx is found in many protostomian phyla and
is considered a primitive stage of cuticle (Rieger 1984). The
large, solitary mucous cells of the episphere resemble similar
cells (many large vacuoles, again lacking a clear opening) in
the notum of aplacophoran molluscs, Solenogastres and
Caudofoveata (e.g. Hoffman 1949). It needs to be shown
whether the larval cuticle already includes chitin, like the
cuticle of the adult Kamptozoa and aculiferan Mollusca
(Salvini-Plawen and Nopp 1974; Jeuniaux 1982; Unger and
Bartolomaeus 2004).
The significance of the various aspects of the nervous
system concerning kamptozoan–molluscan affinities has
been outlined recently by Wanninger et al. (2007). Here, we
note again that the concurrence of an apical ganglion and
true cerebral ganglia, as well as the presence of a trochal ring
in addition to the lateral and ventral cords in the same stages,
reflects the larval–adult mosaic nature of the kamptozoan
Acta Zoologica (Stockholm) 89: 137–148 (April 2008)
creeping larva. Our TEM sections clearly confirm the
tetraneuran condition of the whole (not just serotonergic)
nervous system, which so far has been considered as a
diagnostic character for the Mollusca.
Whereas a ciliated apical organ is found in the overwhelming majority of (non-ecdysozoan) invertebrate larvae
(for details see Wanninger et al. 2007), the frontal organs
have in the past been considered as a kamptozoan apomorphy
(Mariscal 1965; Nielsen 1971, 2005). The frontal organs
are paired basally and fused in more derived forms. Photoreceptive structures, which may be interpreted as cerebral
eyes, have been found in many kamptozoan species within
the frontal organs. Cerebral eyes in Mollusca are restricted
to Gastropoda and Cephalopoda and are unlikely to be a
feature of the molluscan ground pattern (Salvini-Plawen
1981; Haszprunar 2000). In addition, the kamptozoan photoreceptors are of the ciliary type (Woollacott and Eakin 1973),
which in Mollusca is restricted to few, long-planktonic larvae
(e.g. Blumer 1995, 1998). Whereas photoreceptor cells are
probably plesiomorphic for Bilateria or even Metazoa, the
homology of cerebral eyes as organs between phyla remains
doubtful (see recent discussion in Arendt 2003; Gehring
2004; Plachetzki et al. 2005; or Fernald 2006). Interestingly,
the bryozoan (ectoproct) larvae likewise exhibit ciliary
photoreceptors (e.g. Woollacott and Zimmer 1972; Hughes
and Wollacott 1978, 1980; Reed 1988; Reed et al. 1988).
The presence of a paired (respectively symmetrical),
invaginated and contractile, preoral sensory system is shared
with the Solenogastres, where such a structure is known as
‘atrial sensory organ’. The fine structure of the latter is still
poorly known (Haszprunar 1986; reproduced in Scheltema
et al. 1994) and suggests chemoreception as in the case of the
kamptozoan frontal organ. Differences, however, occur in
that the kamptozoan frontal organ is clearly pretrochally
situated, whereas the atrial sense organ of Solenogastres
appears to be a post-trochal structure (Baba 1938; Okuso
2002). The latter is also true for the so-called ‘pedal shield’
(in fact cerebrally innervated) of the Caudofoveata (Chaetodermomorpha), which is a sensory structure rather than a
burrowing organ (Tscherkassky 1989; Scheltema et al. 1994;
Shigeno et al. 2007). Again, direct homology is doubtful.
At first glance, and based on external morphology alone,
the prototroch of the larva of L. murmanica closely resembles
a trochophore with a so-called downstream system, which is
usually correlated with a planktotrophic mode of development (Nielsen 1987, 2001, 2004, 2005). However, no less
than three features do not correspond with this hypothesis:
(1) the gut does not contain any trace of food; (2) the cilia of
the ‘metatroch’ beat downwards as do the cilia of the
‘prototroch’, whereas metatroch cilia beat in the opposite
direction; (3) there is no ciliary band towards the mouth
opening to transport filtered particles. Accordingly, this larva
– although provided with a double ciliary band consisting of
compound cilia – neither has a downstream system nor is
planktotrophic. This result raises doubts on many other cases
© 2007 The Authors
Journal compilation © 2007 The Royal Swedish Academy of Sciences
Acta Zoologica (Stockholm) 89: 137–148 ( April 2008)
Haszprunar and Wanninger • Fine structure of Loxosomella larva (Entoprocta)
throughout the spiralian phyla, where SEM data alone have
been used to infer the presence of a downstream system. The
exact mode of feeding in kamptozoan larvae, as reported by
Jägersten (1964), needs to be investigated to confirm a
downstream mode of feeding.
The condition of the foot of the kamptozoan gliding larva
provides the most significant similarities with basal Mollusca:
Nearly identical to the conditions of the Solenogastres, the
anterior part of the foot is provided with cirri consisting of
from several to many regularly structured cilia. Somewhat
similar organs have been described for several larvae of
Bryozoa as ‘vibratile plume’ (e.g. Reed et al. 1988; Zimmer
and Woollacott 1989, 1993), but these larvae are planktonic
rather than of the benthic creeping-gliding type. The larvae
of Loxosomella and Solenogastres share a so-called pedal
gland, which occupies a considerable volume of the head
region and is continued more posteriorly by differently
structured mucous cells (sole glands). The most exciting
finding, however, is the presence of the intercrossing
dorsoventral muscle fibres – up to now a diagnostic character
of the Mollusca. In summary, the foot of the kamptozoan
larva shares all major characteristics with the foot of basal
molluscs, particularly of the Solenogastres, because the
Caudofoveata (Chaetodermomorpha) entirely lack a pedal
sole and the Polyplacophora show a pedal gland only in the
larval stage (Hammarsten and Runnström 1926).
As outlined above, the gut does not show any trace of larval
feeding. It is interesting to note that the position of the anal
opening exactly corresponds to that of the Polyplacophora
and Monoplacophora, i.e. slightly dorsal to the rear of the foot.
The presence of a single pair of laterally situated protonephridia is typical for spiralian larvae. Since we could not
follow the larvae through metamorphosis, it remains open,
whether the adult protonephridia, the fine-structure of which
somewhat differs from those of the larva (Franke 1993), are
the retained larval ones or of secondary origin.
A clade of Mollusca and Kamptozoa
The present investigation provided significant similarities
between Mollusca and Kamptozoa. Whereas molecular data
up to now have only occasionally contributed significantly
to resolving the interrelationships of the phyla within the
Spiralia or Lophotrochozoa (e.g. Boore and Staton 2002),
phylogenetic considerations and cladistic analyses of
morphological characters revealed a clade ‘Lacunifera’, consisting of both phyla, more than 10 years ago (Bartolomaeus
1993a,b; Haszprunar 1996, 2000; Ax 1999). At present, and
with the new data provided herein, an impressive list of
potential synapomorphies uniting Mollusca and Kamptozoa
can be presented:
1 The division of the body into a cuticularized episphere and
a ciliary–microvillous pedal part.
2 A chitinous cuticle that is not moulted (Unger and
Bartolomaeus 2004).
© 2007 The Authors
Journal compilation © 2007 The Royal Swedish Academy of Sciences
3 A pedal gliding sole with (3a) anteriorly placed cirri, (3b)
a symmetrical/paired pedal gland, which reaches deeply
into the head region of the body, and (3c) a pedal sole with
epidermal mucous cells.
4 Dorsoventral muscles that intercross above the pedal sole
(so far diagnostic for Mollusca).
5 A sinus (tubes that lack endothelia) circulatory system
(Bartolomaeus 1993a,b).
6 An anal opening that opens slightly dorsal of the rear of the
pedal sole.
7 A tetraneurous nervous system (so far diagnostic for
Mollusca) with (7a) pedal commissures, (7b) a buccal
nerve ring, and (7c) an anterior nerve loop (Wanninger
et al. 2007).
8 A complex apical organ consisting of several serotonergic
cells (contrary to most other trochozoans, which usually
only express very few serotonergic cells in the apical organ)
and of two cell types (serotonergic ciliary flask cells and
additional peripheral cells) (Voronezhskaya et al. 2002;
Wanninger et al. 2007).
9 Possibly also the solitary mucous cells in the episphere,
which may correspond to similar structures found in the
notum of Solenogastres.
To summarize, we think that Lacunifera, the clade consisting of Kamptozoa and Mollusca, is currently among
the best documented interrelationships of two metazoan
phyla. Whereas we agree that phylogenetics is always a matter
of probabilities and can never be proven in a strict sense,
we believe it will be hard to overcome this list of distinct
similarities with an alternative hypothesis.
Implications for molluscan origin and the position of the
Solenogastres
Accepting the similarities listed above as synapomorphies of
Mollusca and Kamptozoa, and thus accepting Lacunifera as
a clade, also has implications for the hypothetical ancestor of
the Mollusca. In addition, an accepted sister-group relationship between Kamptozoa and Mollusca also roots character
polarity concerning the phylogenetic position of molluscan
subgroups.
A clade Lacunifera is a direct counter-hypothesis against
the proposed sister-group relationship of the Mollusca to the
Sipuncula (Scheltema 1993, 1996). The latter hypothesis has
already been weakened by cladistic analysis of morphological
characters (Haszprunar 1996, 2000), developmental data
(Wanninger et al. 2005), and by molecular analyses (Boore
and Staton 2002). We therefore regard this hypothesis as no
longer valid.
Maybe the most important point is the deduced hypothesis
that a molluscan-like pedal sole with pedal gland was already
a feature of the premolluscan (i.e. sinusoidean) ancestor and
not an evolutionary novelty within the Mollusca. Accordingly, the hypothesis of Salvini-Plawen (1972, 1981, 1985,
1991, 2003) of a plesiomorphic turbellaria-like (cerebrally
Fine structure of Loxosomella larva (Entoprocta) • Haszprunar and Wanninger
and pedally innervated) gliding sole and the basic division of
the Mollusca into Scutopoda (with a retained cerebral part)
and Adenopoda (with a retained pedal part) should no longer
be upheld (see also Salvini-Plawen 2006).
A direct sister-group relationship of the acoelomate
Kamptozoa and the coelomate (but not eucoelomate; cf.
Salvini-Plawen and Bartolomaeus 1995; Haszprunar 1996;
for discussion) Mollusca renders homology of the molluscan
coelomic cavities (i.e. the gonopericardial system) and the
coelom of Sipuncula or Annelida unlikely.
The present results also shed additional light on the
assumed basic position of the Solenogastres (Salvini-Plawen
and Steiner 1996; Haszprunar 2000) within the Mollusca:
only this molluscan group (still) exhibits a pedal sole with
anterior cirri, only this taxon shows the distinct solitary
mucous cells in the notum.
Outlook
The present findings shed light on several anatomical
features of the kamptozoan creeping larva. However, several
questions, and in particular the variation of most characters
within the Kamptozoa, remain to be studied. It is time to pay
more attention to this neglected group.
Acknowledgements
We thank the Marine Biological Station in Kristineberg for
hospitality. Eva Lodde (ZSM) and the late Alenka Kerin
(Department Biology I of LMU Munich) provided technical
support during preparation, Prof. Dr M. Starck (Department
Biology II of LMU Munich) and his team were responsible
for the TEM facilities. Marianne Müller (ZSM) helped
with the photographic and digital work. We also thank two
anonymous referees for detailed and very valuable comments
on the draft of the typescript. This work was supported
by funds from the Danish Research Agency (FNU; grant
21-04-0356 and 272-05-0174 to A.W.) and the European
Commission (ARI-programme, 5th framework, to A.W.).
References
Arendt, D. 2003. Evolution of eyes and photoreceptor cell types. –
International Journal of Developmental Biology 47: 563–571.
Ax, P. 1999. Das System der Metazoa. II. Stuttgart 383 pp, Gustav
Fischer.
Baba, K. 1938. The later development of a solenogastre, Epimenia
verrucosa (Nierstrasz). – Journal of the Department of Agriculture,
Kyushu Imperial University 6: 21–40.
Bartolomaeus, T. 1993a. Die Leibeshöhlenverhältnisse und
Verwandtschaftsbeziehungen der Spiralia. – Verhandlungen der
Deutschen Zoologischen Gesellschaft 86: 42.
Bartolomaeus, T. 1993b. Die LeibeshÖhlenverhÄltnisse und Nephridialorgane der Bilateria–Ultrastruktur, Entwicklung und Evolution,
Habilitationsschrift. University of Göttingen, Göttingen.
Blumer, M. J. F. 1995. The ciliary photoreceptors in the teleplanic
Acta Zoologica (Stockholm) 89: 137–148 (April 2008)
veliger larvae of Smaragdia sp. & Strombus sp. (Mollusca,
Gastropoda). – Zoomorphology 115: 73 –81.
Blumer, M. J. F. 1998. Alterations of the eyes of Carinaria lamarcki
(Gastropoda, Heteropoda) during the long pelagic cycle. –
Zoomorphology 118: 183–194.
Boore, J. L. and Staton, J. L. 2002. The mitochondrial genome of
the sipunculid Phascolopsis gouldii supports its association with
Annelida rather than Mollusca. – Molecular Biology and Evolution
19: 127–137.
Eeckhaut, I. and Jangoux, M. 1993. Integument and epidermal
sensory structures of Myzostoma cirriferum (Myzostomida). –
Zoomorphology 113: 33 –45.
Emschermann, P. 1969. Ein Kreislauforgan bei Kamptozoen. –
Zeitschrift für Zellforschung 97: 576–607.
Emschermann, P. 1982. Les Kamptozoaires. Ètat actuel de nos connaissance sur leur anatomie, leur développement, leur biologie
et leur position phylogènètique. – Bulletin de la Societé Zoologique
de France 107: 317–344.
Fernald, R. D. 2006. Casting a genetic light on the evolution of eyes.
– Science 313: 1914 –1918.
Franke, M. 1993. Ultrastructure of the protonephridia in Loxosomella fauveli, Barentsia matsushimana and Pedicellina cernua.
Implications for the protonephridia in the ground pattern of the
Entoprocta (Kamptozoa). – Microfauna Marina 8: 7–38.
Franzén, A. 1970. Morfologi och larvutveckling hos Entoprocta.
– Svensk Naturvetenskap 1970: 131–141.
Friedrich, S., Wanninger, A., Brückner, M. and Haszprunar, G.
2002. Neurogenesis in the mossy chiton, Mopalia muscosa (Gould)
(Polyplacophora): evidence versus molluscan metamerism. – Journal
of Morphology 253: 109–117.
Gehring, W. J. 2004. Historical perspective on the development and
evolution of eyes and photoreceptors. – International Journal of
Developmental Biology 48: 707–717.
Hammarsten, O. D. and Runnström, J. 1926. Zur Embryologie von
Acanthochiton discrepans (Brown). – Zoologische Jahrbücher,
Abteilung Anatomie 47: 261–318.
Haszprunar, G. 1986. Feinmorphologische Untersuchungen an
Sinnesstrukturen ursprünglicher Solenogastres (Mollusca). –
Zoologischer Anzeiger 217: 345– 362.
Haszprunar, G. 1996. The Mollusca: coelomate turbellarians or
mesenchymate annelids?. In: Taylor, J. D. (Ed.): Origin and
Evolutionary Radiation of the Mollusca, pp. 1–28. Oxford University
Press, Oxford.
Haszprunar, G. 2000. Is the Aplacophora monophyletic? A cladistic
point of view. – American Malacological Bulletin 15: 115–130.
Hoffman, S. 1949. Studien über das Integument der Solenogastres,
nebst Bemerkungen über die Verwandtschaft zwischen den
Solenogastres und Placophoren. – Zoologiska Bidrag Fran Uppsala
27: 293–427.
Hughes, R. L. Jr and Wollacott, R. M. 1978. Ultrastructure of
potential photoreceptor organs in the larva of Scrupocellaria bertholetti (Bryozoa). – Zoomorphology 91: 225– 234.
Hughes, R. L. Jr and Woollacott, R. M. 1980. Photoreceptors of
bryozoan larvae (Cheilostomata, Cellularioidea). – Zoologica
Scripta 9: 129–138.
Jägersten, G. 1964. On the morphology and reproduction of
entoproct larvae. – Zoologiska Bidrag Fran Uppsala 36: 295 –314,
plates 1–3.
Jeuniaux, C. 1982. Composition chimique comparée des formations
squelettiques chez les Lophophoriens et les Endoproctes. –
Bulletin de la Societé Zoologique de France 107: 233–249.
Marcus, E. 1939. Bryozoarios marinhos brasileiros III. – Boletim de
Faculdad Filosofia et Ciencia, Sao Paulo, Zoologia 3: 11–353.
© 2007 The Authors
Journal compilation © 2007 The Royal Swedish Academy of Sciences
Acta Zoologica (Stockholm) 89: 137–148 ( April 2008)
Haszprunar and Wanninger • Fine structure of Loxosomella larva (Entoprocta)
Mariscal, R. N. 1965. The adult and larval morphology and life
history of the entoproct Barentsia gracilis (M. Sars 1835). – Journal
of Morphology 116: 311–338.
Maslakova, S. A., Martindale, M. Q. and Norenburg, J. L. 2004.
Vestigial prototroch in a basal nemertean, Carinoma tremaphoros
(Nemertea; Palaeonemertea). – Evolution and Development 6:
219–226.
Nielsen, C. 1967. Metamorphosis of the larva of Loxosomella
murmanica (Nilus) (Entoprocta). – Ophelia 4: 85–89.
Nielsen, C. 1971. Entoproct life-cycles and the entoproct/ectoproct
relationship. – Ophelia 9: 209– 341.
Nielsen, C. 1987. Structure and function of metazoan ciliary bands
and their phylogenetic significance. – Acta Zoologica (Stockholm)
68: 205–262.
Nielsen, C. 2001. Animal Evolution. Interrelationships of the Living
Phyla, 2nd edn., x + 563, pp. Oxford University Press, Oxford.
Nielsen, C. 2002a. Phylum Entoprocta. In: Young, C. M., Sewell,
M. A. and Rice, M. E. (Eds.): Atlas of Marine Invertebrate Larvae,
pp. 397– 409. Academic Press, San Diego.
Nielsen, C. 2002b. The phylogenetic position of Entoprocta,
Ectoprocta, Phoronida and Brachiopoda. – Integrative and Comparative Biology 42: 685– 691.
Nielsen, C. 2004. Trochophora larvae: cell-lineages, ciliary bands,
and body regions. 1. Annelida and Mollusca. – Journal of Experimental Zoology (Molecular Development and Evolution) 302B:
35–68.
Nielsen, C. 2005. Trochophora larvae: cell-lineages, ciliary bands
and body regions. 2. Other groups and general discussion. –
Journal of Experimental Zoology (Molecular Development and
Evolution) 304B: 401– 447.
Okusu, A. 2002. Embryogenesis and development of Epimenia babai
(Mollusca Aplacophora). – The Biological Bulletin 203: 87–103.
Plachetzki, D. C., Serb, J. M. and Oakley, T. H. 2005. New insights
into the evolutionary history of photoreceptor cells. – Trends in
Ecology and Evolution 20: 465– 467.
Reed, C. G. 1988. Organization of the nervous system and sensory
organs in the larva of the marine bryozoan Bowerbankia gracilis
(Ctenostomata: Vesiculariidae): functional significance of the
apical disc and pyriform organ. – Acta Zoologica. (Stockholm) 69:
177–194.
Reed, C. G., Ninos, J. M. and Woollacott, R. M. 1988. Bryozoan
larvae as mosaics of multifunctional ciliary fields: ultrastructure of
the sensory organs of Bugula stolonifera (Cheilostomata: Cellularioidea). – Journal of Morphology 197: 127–145.
Reynolds, E. S. 1963. The use of lead citrate at high pH as an electron opaque stain in electron microscopy. – Journal of Cell Biology
17: 208–212.
Rieger, R. M. 1984. Evolution of the cuticle in the lower Eumetazoa.
In: Bereiter-Hahn, J., Matoltsy, A. G. and Richards, K. S. (Eds):
Biology of the Integument, Vol. I. Invertebrates, pp. 389–399.
Springer Verlag, Heidelberg – New York.
Rouse, G. W. 1999. Trochophore concepts: ciliary bands and the
evolution of larvae in spiralian Metazoa. – Biological Journal of the
Linnean Society 66: 411– 464.
Salvini-Plawen, L. V. and Bartolomaeus, T. 1995. Mollusca, mesenchymata with a ‘coelom’. In: Lanzavecchia, G., Valvassori, R. and
Candia Carnevali, M. D. (Eds): Body Cavities: Function and
Phylogeny, Selected Symposia & Monographs 8: pp. 75–92. Mucchi,
Modena.
Salvini-Plawen, L. V. 1972. Zur Morphologie und Phylogenie der
Mollusken: Die Beziehung der Caudofoveata und der Solenogastres
als Aculifera, als Mollusca und als Spiralia. – Zeitschrift für Wissenschaftliche Zoologie 184: 205–394.
© 2007 The Authors
Journal compilation © 2007 The Royal Swedish Academy of Sciences
Salvini-Plawen, L. V. 1981. On the origin and evolution of the
Mollusca. Origine dei grandi phyla dei Metazoi. – Atti Dei Convegni
Lincei (Roma) 49: 235–293.
Salvini-Plawen, L. V. 1985. Early evolution and the primitive groups.
In: Trueman, E. R. and Clarke, M. R. (Eds): The Mollusca. Vol.
10: Evolution, pp. 59–150. Academic Press, London.
Salvini-Plawen, L. V. 1991 (‘1990’). Origin, phylogeny and classification of the phylum Mollusca. – Iberus 9: 1–33.
Salvini-Plawen, L. V. 2003. On the phylogenetic significance of the
aplacophoran Mollusca. – Iberus 21: 67–97.
Salvini-Plawen, L. V. 2006. The significance of the Placophora for
molluscan phylogeny. – Venus (Japanese Journal of Malacology) 65:
1–17.
Salvini-Plawen, L. V. and Nopp, H. 1974. Chitin bei Caudofoveata
(Mollusca) and die Ableitung ihres Radulaapparates. – Zeitschrift
für Morphologie der Tiere 77: 77–86.
Salvini-Plawen, L. V. and Steiner, G. 1996. Synapomorphies and
plesiomorphies in higher classification of Mollusca. In: Taylor, J.
D. (Ed.): Origin and Evolutionary Radiation of the Mollusca, pp.
29 –51. Oxford University Press, Oxford.
Scheltema, A. H. 1993. Aplacophora as progenetic aculiferans and
the coelomatic origin of mollusks as the sister taxon of Sipuncula.
– Biology Bulletin 184: 57–78.
Scheltema, A. H. 1996. Phylogenetic position of Sipuncula,
Mollusca and the progenetic Aplacophora. In: Taylor, J. D. (Ed.):
Origin and Evolutionary Radiation of the Mollusca, pp. 53–58.
Oxford University Press, Oxford.
Scheltema, A. H., Tscherkassky, M. and Kuzirian, A. M. 1994.
Aplacophora. In: Harrison, F. W. and Kohn, A. J. (Eds): Microscopic Anatomy of Invertebrates, Vol. 5: Mollusca I: Aplacophora,
Polyplacophora, and Gastropoda, pp. 13 –54. Wiley-Liss, New York.
Sensenbaugh, T. 1986. Ultrastructure of apical sense organs in
aquatic invertebrate larvae. – Acta Universitatis Uppsala (Comprehensive Summariea et Abstracta of Dissertationa in Science) 34:
1–31.
Sensenbaugh, T. 1987. Ultrastructural observations on the larva of
Loxosoma pectinaricola Franzén (Entoprocta, Loxosomatidae). –
Acta Zoologica (Stockholm) 68: 135–145.
Shigeno, S., Sasaki, T. and Haszprunar, G. 2007. The central nervous
system of Chaetoderma japonica (Mollusca, Caudofoveata):
implications for diversified cerebral cords in primitive molluscs. –
Journal of Morphology (In press).
Spurr, A. R. 1969. A low-viscosity epoxy resin embedding medium
for electron microscopy. – Journal of Ultrastructure Research 26:
31–43.
Tscherkassky, M. 1989. Pedal shield or oral shield in Caudofoveata
(Mollusca, Aculifera). – Abstracts of the 10th International Malacological Congress at Tübingen 1989, p. 254. UNITAS, Tünginen.
Unger, A. and Bartolomaeus, T. 2004. Evolution of dorsal chitincuticles within entoprocts & molluscs. – Abstracts der 97. Jahresversammlung der Deutsche Zoologische Gesellschaft at Rostock 2004,
p.77.
Voronezhskaya, E. E., Tyurin, S. A. and Nezlin, L. P. 2002. Neuronal
development in larval chiton Ischnochiton hakodadensis (Mollusca:
Polyplacophora). – Journal of Comparative Neurology 444: 25–
38.
Wanninger, A., Koop, D., Bromham, L., Noonan, E. and Degnan, B. M.
2005. Nervous and muscle system development in Phascolion strombus
(Sipuncula). – Development, Genes and Evolution 215: 509–518.
Wanninger, A., Fuchs, J. and Haszprunar, G. 2007. The anatomy of the
serotonergic nervous system of an entoproct creeping-type larva
supports a mollusc-entoproct clade. – Invertebrate Biology in press.
Woollacott, R. M. and Eakin, R. M. 1973. Ultrastructure of a potential
Fine structure of Loxosomella larva (Entoprocta) • Haszprunar and Wanninger
photoreceptor organ in the larva of an entoproct. – Journal of
Ultrastructure Research 43: 412– 425.
Woollacott, R. M. and Zimmer, R. L. 1972. Fine structure of a
potential photoreceptor organ in the larva of Bugula neritina
(Bryozoa). – Zeitschrift für Zellforschung 123: 458–469.
Zimmer, R. L. and Woollacott, R. M. 1989. Larval morphology of
Acta Zoologica (Stockholm) 89: 137–148 (April 2008)
the bryozoan Watersipora arcuata (Cheilostomata: Ascophora). –
Journal of Morphology 199: 125–150.
Zimmer, R. L. and Woollacott, R. M. 1993. Anatomy of the larva of
Amathia vidovici (Bryozoa: Ctenostomata) and phylogenetic
significance of the vesiculariform larva. – Journal of Morphology
215: 1–29.
© 2007 The Authors
Journal compilation © 2007 The Royal Swedish Academy of Sciences