A M . ZOOLOCIST, 9:803-812 (1969).
Possible Boring Structures of Sipunculids
MARY E. RICE
Division of Worms, Department of Invertebrate Zoology,
Museum of Natural History, Smithsonian
Institution,
Washington, D. C. 20560
SYNOPSIS. At the present time there is no experimental evidence which links the
supposed boring activities of sipunculids to a specific organ or structure. Structures
which have been speculatively associated in the literature with boring are: hooks and
spines of the introvert, cuticular papillae with associated epidermal glands, anterior
and posterior horny shields, and anterior calcareous shields. In this review these
structures are described as they occur in five representative species of sipunculids
collected by the author from calcareous rock in the Indian Ocean or the Caribbean
Sea. The five species are: Pliascolosoma antillarum Grube and Oersted, Phascolosoma
dentigerum (Selenka and de Man) , Paraspidosiphon steenstrupi (Diesing), Lithacrosiphon gurjanovae Murina, and Cloeosiphon aspergillum (Quatrefages). Localities of
collections are cited, habitats and burrows are described, and the behavior of the
animals as observed in the field and laboratory is noted. In view of the morphology of
the possible boring structures and in light of observations on habitats and behavior,
the possible roles of the structures in boring activities are discussed.
Highly organized horny shields are present at the anterior and posterior extremities
of the trunk or Paraspidosiphon steenstrupi, whereas anterior calcareous shields are
characteristic of Cloeosiphon aspergillum and Lithacrosiphon gurjanovae. Papillae and
epidermal glands are present in all five of the species but these are most highly
developed in Phascolosoma dentigerum and P. antillarum. Of the species considered,
only P. antillarum lacks hooks on the introvert.
Because of the position of the animal within the rock with anterior end directed
toward the mouth of the burrow, it is assumed that the anterior shields and the hooks
of the introvert play no significant role in the formation of the burrow. However,
the rigid papillae of the trunk and the thickened posterior shield, if rubbed against
the wall of the burrow, presumably could be utilized in the mechanical attrition of the
more friable rock, whereas the secretory products of the numerous epidermal
glands might be implicated in the chemical dissolution of the harder substrates.
T h e sipunculids form a common and
significant component of the communities
which inhabit the calcareous rock of the
tropical and subtropical seas. They may be
found dwelling within burrows in beach
rock, coral rock, the dead portions of coral
colonies, or in coral rubble, ranging from
intertidal to subtidal depths. Although it
has not been demonstrated that these animals are capable of boring, it has been
generally presumed that the worms do
form their own burrows because of the
exact fit of the body within the hole and
the smooth linings of the walls of the cavities.
A t t h e p r e s e n t time there is no experimental evidence which links the supposed
boring activities of sipunculids to a specific
organ or structure, nor is there any evid e n c e i n t h e literature to elucidate wheth-
Collections in the Indian Ocean were made on
Cruise B of the Te Vega Expeditions, supported by
NSF Grant No. G-17465.
er boring might be accomplished by
mechanical or chemical means. Structures
which have been speculatively associated
I wish to express my appreciation to Dr J W.
Pierce and Mr. Roger Hughes, Department of Paleobiology, Smithsonian Institution, for aid in the
characterization of the calcareous substrates and to
Dr. N. Watabe and Dr. Karl Wilbur, Departmerit of Zoology, Duke University, for advice and
help in studies on the calcareous cone of Lithacro-
;
activities a r e : h o o k s a n d s p i n e s
r
. °
. ,
...
introvert, cuticular papillae, epi •
dermal glands, anterior
and
posterior
shields and
anterior
calcareous
h o r n y
.
,
.
,_„,
shields.
Some
authors
(Sluiter,
1891;
siphon gurjanovae.
Shipley, 1890) have proposed that the epi-
w k h b o r
.
o f
803
,
t h e
804
MARY E. RICE
FIG. 1. Rock-dwelling sipunculids. From left to gillum was collected from coralline limestone in the
right: Phascolosoma antillarum, Phascolosoma den- Maldive Islands, Indian Ocean; the other four
species from calcareous rock in various localities in
tigerum, Paraspidosiphon steenstrupi, Lithacrosiphon gurjanovae, Cloeosiphon aspergillum. C. asper- the Caribbean Sea. Scale 5 mm.
dermal glands produce a secretion effective
in the chemical dissolution of the calcareous substrate, whereas others (Otter, 1937;
Gardiner, 1903; Yonge, 1963) have suggested that the hooks, papillae, or shields
may be utilized for the mechanical
abrasion of the substrate.
For this review the possible boring structures are described for five representative
species of sipunculids collected by the author from burrows in calcareous rock in
the Indian Ocean or the Caribbean Sea.
Observations are included on the localities
from which the animals were collected,
their habitats, and behavior. The species
to be considered are: Phascolosoma antillarum Grube and Oersted, Phascolosoma
dentigerum (Selenka and de Man), Paraspidosiphon steenstrupi (Diesing), Lithacrosiphon gurjanovae Murina, and Cloeosiphon aspergillum (Quatrefages) (Fig. 1).
Paraspidosiphon steenstrupi possesses highly organized horny shields at the anterior
and posterior ends of the trunk while L.
gurjanovae and C. aspergillum are characterized by anterior calcareous shields and
no posterior shields. Shields are absent in
the species of Phascolosoma, but cuticular
papillae and associated epidermal glands
are highly developed. With the exception
of P. antillarum, all of the species considered possess hooks on the introvert. In
view of the morphology of these possible
boring structures and in light of the observations on the habitats and behavior of the
animals, the possible roles of the structures in boring activities are discussed. In
addition, an incidental observation is reported on the penetration of juvenile
sipunculids into cavities of calcareous rock
with the subsequent establishment of bur.
rows.
BORING STRUCTURES OF SIPUNCULIDS
COLLECTION OF ANIMALS
Localities
Rock-dwelling sipunculids were collected from a total of 28 stations in the Caribbean and Straits of Florida and from a
total of 6 stations in the Indian Ocean.
PhascolosQma antillarum was collected
from 21 stations in the Caribbean, Paraspidosiphon steenstrnpi from 7 stations, and
Lithacrosiphon gurjanovae from 5 stations. Cloeosiphon aspergillum was collected from 5 stations in the Indian Ocean and
Phascolosoma dentigerum from 4 stations
in the Indian Ocean and 26 stations in the
Caribbean. Localities of the Caribbean
stations were in Puerto Rico, Barbados,
Venezuela, Curacao, Jamaica, Florida, and
Bimini. All of the stations in the Indian
Ocean were located in the Maldive Islands
in the atolls of Male, Fadiffolu, Tiladummati, North Malosmadulu, and Addu.
Other species of sipunculids also occurred
in the calcareous rock at these stations, but
the five species selected for consideration
805
here were the most abundant, with the
exception of a few species of small Aspidosiphon and Paraspidosiphon as yet unidentified.
Habitats
The various habitats from which animals were collected included calcified mangrove reefs, fossil coral reefs, beach rock,
recent coralline limestone, calcarenite
boulders, coral rubble, and the dead portions of living coral colonies. Most of the
collections were made intertidally, but a
few were subtidal at depths of two to five
feet. The texture of the rocks varied from
the exceedingly friable mangrove reef and
cemented boulders to the very hard structure of the recent coralline limestone. The
rocks in which the sipunculids were found
were without exception predominantly
calcareous, although in the beach rock and
other calcarenites quartz grains constituted a minor component. Selected samples
of rocks from 7 stations in the Caribbean
TABLE 1. Description of selected habitats.
Locality
Enrique Eeef,
near La Pargucra,
Puerto Rico
Punta de Cerro
Gordo, Puerto Eico
Cayo Turrumote
near La Pargucra,
Puerto Eico
Boqucron Bay,
Puerto Eico
Cabo Eojo,
Puerto Eico
Six Men's Bay,
Barbados
Bath, Barbados
Description of Habitat
Recent coralline limestone, aragonite with
some high magnesium calcite on surface,
intertidal and subtidal.
Cemented calcarenite, mostly aragonite,
some high magnesium calcite, heavy minerals, iron-stained, glauconite, outcrop on
sandy beach, exposed to heavy surf, intertidal.
Recent coralline limestone boulders, aragonite with calcite and aragonito on surface,
intertidal and subtidal.
Pitted coralline limestone boulders, mostly
aragonite with some high magnesium calcite, microstructure of coral blurred, subtidal.
Pitted c-alcarenite boulders, calcite with
some aragonite, some smoky quartz grains,
intertidal.
Beach rock, coarse-grained biocalcarenite,
some quartz grains, cemented together by
calcite, intertidal.
Highly pitted coralline limestone, mostly
calcite, some aragonite, some quartz grains,
microstructure of coral nearly destroyed,
subtidal.
Species Collected
Phascolosoma antillarum
Phascolosoma dentigerum
Paraspidosiphon steenstrupi
Phascolosoma antillarum
Phascolosoma dentigerum
Phascolosoma antillarum
Phascolosoma dentigerum
Paraspidosiphon steenstrupi
Lithacrosiphon gurjanovae
Phascolosoma antillarum
Phascolosoma dentigerum
Paraspidosiphon steenstrupi
Lithacrosiphon gurjanovae
Phascolosoma antillarum
Phascolosoma dentigerum
T'hascolosoma antillarum
Phascolosoma dentigerum
Paraspidosiphon, steenstrupi
Lithacrosiphon gurjanovae
Phascolosoma antillarum
806
MARY E. RICE
were tested by x-ray diffraction for the
relative content of calcite and aragonite
(Table 1). In the more weathered rocks
calcite was predominant, whereas in the
hard coral rock with the structure of the
coral still intact, aragonite was the predominant form. The surface of the rocks
was usually covered by heavy growths of
epiphytes, frequently pitted, and often
fenestrated by boring sponges.
Although the five species of sipunculids
have been collected exclusively from predominantly calcareous rocks, no speciespreference has been demonstrated for
rocks of differing texture and hardness. All
of the species have been found in rocks of
both friable and compact textures, containing calcite as well as aragonite, and
located in both intertidal and subtidal
regions (Table 1). Frequently three or
more species occurred within the same
rock in adjacent burrows.
Burrows
Observations have shown that the burrows of the five species of sipunculids considered here may occur at any angle to the
surface of the rock; they may be straight or
exceedingly sinuous and winding. The
lining is nearly always very smooth and
occasionally it is marked by an unexplained yellow or black coloration. In
rocks in which quartz grains are a minor
constituent, the siliceous particles project,
unchanged, into the cavity of the burrow.
Each burrow has only one opening and
this may occur on any surface of the rock:
upper, beneath an overhang or on a lower
surface which is lying on the sand. A burrow directed from one surface toward another surface of a rock will turn abruptly
before reaching the second surface. Only
rarely, in densely populated rocks, do burrows actually open into one another. The
mouth of the burrow is oval or circular,
frequently with a smaller diameter than
that of the remainder of the burrow. The
diameter of the burrow proper is approximately 1/9 to 2/3 that of the animal after its
removal from the burrow.
FIG. 2. Burrows of sipunculids in coralline limestone. Rocks were broken open to expose burrows
and animals removed, a. Burrow of Phascolosoma
antillarum. From Cayo Turrumote, Puerto Rico.
Scale 5 mm. b. Burrows of Cloeosiphon aspergillum.
Cross section and longitudinal section. From North
Malosmadulu, Maldive Islands. Scale 5 mm.
BORING STRUCTURES OF SIPUNCULIDS
807
The burrows of Phascolosoma denligerum are sinuous, long, and narrow,
while those of P. antillanim are usually
straight and wide with a typically cupshaped blind end. (Fig. 2a) Lithacrosiphon gurjanovae inhabits burrows that are
short and straight, and when within Porites rubble they appear more or less parallel. The burrows of Cloeosiphon aspergillum are long and winding, extending deep
into the hard coral rock where no other
living organisms penetrate (Fig. 2b).
POSSIBLE BORING STRUCTURES
Hooks and Spines
The hooks of sipunculids are small
hardened surface ornamentations, usually
arranged in rings or rows at the anterior
end of the introvert. Varying in size among
different species, the hooks are somewhat
flattened structures, broad at the attached
base and narrowing at the distal end with
a curved terminal portion. Typically the
hooks are characterized by clear cavities
and thickened ridges. Although there are
some exceptions, such as Phascolosoma antillarum, most of the rock-dwelling sipunculids possess hooks on the introvert.
In Phascolosoma dentigerum there are
16 to 22 rings of hooks on the anterior
introvert (Fig. 3), in Paraspidosiphon
steenstrupi 20 to 44, in Cloeosiphon aspergillum 20 to 25 rings, and in Lithacrosiphon
gurjanovae 30 rings, the remainder of the
introvert being covered with irregularly arranged hooks. Frequently the more posterior rings of hooks are incomplete, due to an
apparent loss of hooks. The hooks of the
4 species vary in shape and extent of the
clear areas or cavities. Phascolosoma dentigerum has a characteristic central clear
streak and, on the convex side, a basal
clear triangular area (Fig. 4a). In other
species these areas are not so distinctive
and in P. steenstrupi they are merged into
a single broad clear area. The hooks of P.
steenstrupi and Cloeosiphon aspergillum
are bifid with terminal and subterminal
points, whereas in the hooks of Phascolosoma dentigerum the subterminal
FIG. 3. Anterior introvert of Phascolosoma dentigerum showing rings of hooks. Note tentacles at
end of introvert. Scale 0.2 mm.
point may be either present or absent
(Fig. 4a, b, e). In Lithacrosiphon gurjanovae the hooks on the anterior introvert
have two points while the posterior hooks
have only one point (Fig. 4g, i). The posterior introvert of P. steenstrupi is covered
with scattered spines which are distinguished from the hooks by their more
cylindrical rather than flattened shape and
the fact that they are straight rather than
curved terminally (Fig. 4c). The hooks of
C. aspergillum are the largest of the five
species, being approximately 0.10 mm in
height, whereas the one-pointed hooks of
L. gurjanovae are smallest, measuring only
0.032 mm in height.
Papillae
The papillae of sipunculids are cuticular
protuberances of varying sizes and shapes
which are distributed over the trunk and
introvert in a characteristic pattern. Associated with each papilla and projecting
upward into it is a glandular epidermal
organ which usually opens to the exterior
808
MARY E. RICE
die region of the trunk. The platelets of
the smaller papillae are closely packed, decreasing in size toward the periphery. In
the larger papillae the platelets are thicker
and in the area immediately surrounding
the central pore they may be somewhat
smaller.
The papillae of P. antillarum are low,
rounded, darkly pigmented, and numerous
over the entire body, but are largest and
most closely packed at the posterior extremity and in a ventral region at the base of
the introvert (Fig. 1). The platelets are
relatively large, distinct, and nearly uniform in size (Fig. 5a). They may be scattered throughout the cuticle, among the
papillae as well as in concentrations on the
papillae. On the introvert the papillae are
further separated and of a pointed, conical
form.
The largest papillae of P. steenstrupi are
localized near the anterior and posterior
shields and attain a height of 0.4 mm.
FIG. 4. Hooks, spines, and papillae of the introPlatelets are closely packed over the survert, a. Hook o£ Phascolosoma dentigerum. b. Hook
face of the papilla and four or more surfrom anterior introvert of Paraspidosiphon steenstrupi. c. Spine from posterior introvert of P. steen- round the central opening. Small cylindristrupi. d. Papilla among rows of hooks. P. steencal papillae between the rows of hooks are
slrupi. Optical section, e. Hook of Cloeosiphon as- half the height of the hooks (Fig. 4d).
pergillum. f. Papilla among rows of hooks. C.
The papillae of C. aspergillum are denaspergillum. Optical section, g. Hook from anterior
sely aggregated at the base of the anterior
introvert of Lithacrosiphon gurjanovae. h. Papilla
among hooks of introvert, L. gurjanovae. Optical
shield where they measure up to 0.10 mm
section, i. Hook from posterior introvert of
in height and are cylindrical in shape. In
Lithacrosiphon gurjanovae.
the middle of the body they are 0.08 mm
through a pore at the apex of the papilla. high and toward the posterior they become
Embedded within the cuticular covering oE progressively shorter and wider. The
the papilla are small horny granules or platelets are arranged in two rows at the
thickenings designated as platelets. The periphery of the papillae and are also scatplatelets give to the papilla a rigidity and tered over the non-papillated portion of
the cuticle. Small tubular papillae with
roughness.
In P. dentigerum there are prominent, horny teeth occur on the introvert between
darkly pigmented, conical papillae local- the rows of hooks (Fig. 4f).
ized at the posterior extremity of the aniIn L. gurjanovae the papillae are relamal and in a dorsal area at the base of the tively inconspicuous, rounded, with a cenintrovert where they are usually pointed in tral opening around which small platelets
a backward direction (Fig. 1). The papil- are concentrically arranged. The platelets
lae become smaller and less concentrated are somewhat larger at the periphery.
toward the anterior introvert and the mid- Small tubular papillae between the rows of
F1G. 5. Papillae and shields. Scale equals 0.2 mm
unless otherwise marked, a. Papillae of Phascolosoma antillarum. Surface view. Note cuticular
platelets (small dark bodies) and the central pores
of the papillae, b. Sagittal section of papilla of
Phascolosoma antillarum showing platelets (pi) of
the cuticle (cu) and the elongated epidermal gland
cells (gl) with secretory granules, extending
BORING STRUCTURES OF SIPUNCULIDS
toward the apex of the papilla. Epon-embedded.
1/i. Stained with methylene blue and Azure II. c.
Posterior shield of Paraspiclosiphon steenstrupi. A.
Anterior calcareous shield of Cloeosiphon aspergillum. e. Anterior shield of Lilhacrosiphon gurjanovae. Shield is comprised of a calcareous cone cov-
809
ered by a ribbed and thickened cuticular sheath, f.
Posterior extremity of a specimen of Phascolosoma
antillarum in which the papillae have been lost.
Xote in the white denuded area a few small
circular papillae, apparently in the process of regeneration.
810
MARY E. RICE
hooks reach a height y3 that of the hook
(Fig. 4h).
Epidermal Glands
In sectioned material, the epidermal
glands of P. antillarum are observed to be
multicellular, all of the gland cells opening
to the exterior through a common pore at
the apex of the papilla (Fig. 5b). The
secretory product takes the form of granules varying from fine, densely stained granular masses to large granules tightly
packed within the cell boundaries and
lightly stained. The granules react positively to the periodic acid-Schiff stain, but
show no evidence of metachromasia when
stained with methylene blue and azure II.
No information is available on the epidermal glands of the other species.
Shields
Paraspidosiphon steenstrupi possesses
well-defined horny shields at both the anterior and posterior extremities of the
trunk (Fig. 1). Anteriorly the shield is on
the dorsal side of the trunk, displacing the
introvert to a ventral position. The anterior shield is an oval, horny, thickened, cuticular structure comprised of many large,
raised, polygonal plates. Frequently white
calcareous granules are attached to the anterior shield. The posterior shield encompasses the posterior extremity of the trunk
and may be hemispherical, knob-like, or
flat, depending on the state of contraction
(Fig. 5c). The plates, similar to those of
the anterior shield, are arranged in concentric rings in the center of the shield and
in radial striations peripherally.
The anterior shield of C. aspergillum is
a rounded or knob-shaped calcareous structure which surrounds the anterior end of
the trunk and the centrally located introvert (Fig. 5d). The shield is composed of
many small polygonal calcareous plates,
arranged in spiral rows. At the center of
each plate is a small, deep brown, circular
pore of the underlying epidermal gland.
There is no posterior shield.
The shield of L. gurjanovae is a hard
calcareous cone at the anterior end of the
trunk which displaces the introvert ventrally (Fig. 5e). The cone is covered by a
thickened, horny, cuticular sheath, ribbed
with longitudinal ridges and grooves running from the base of the cone to the
apex. Embedded in this cuticular covering
are numerous yellowish platelets of irregular size and shape, which become smaller
as well as lighter in color toward the apex
of the cone. In sectioned material the
platelets appear as distinct non-cellular
bodies composed of a homogeneous material which is arranged in a concentric lamellar pattern. The cone itself is situated between two layers of cuticle: the outer thickened layer and thin inner layer which
remains in close contact with the epidermis. The inner layer covers the anterior
end of the trunk, projecting upward into
the basal concavity of the cone. Recent
electron-diffraction studies by Dr. N.
Watabe of Duke University have revealed
that the cone is composed of aragonite crystals. Frequently the anterior shields of this
species are completely covered by a white
encrusting material, believed to be a calcareous alga.
DISCUSSION AND CONCLUSIONS
Observations on behavior
Before an assessment is attempted of the
possible roles of the described structures
in boring activities, it will be of value to
consider some observations on the position
and behavior of living animals within the
burrows.
Without exception, all specimens collected from rocks were situated in their
burrows with anterior ends directed
toward the openings of the burrow. Observations on animals maintained in the laboratory within rocks have shown that P.
dentigerum frequently extends the introvert over the surface of the rock, engulfing
small particles of sand and debris. Phascolosoma antillarum opens its welldeveloped tentacular crown at the mouth
of the burrow and presumably feeds by
BORING STRUCTURES OF SIPUNCULIDS
means of the ciliary activity of the tentacles.
Examination of movements within a
burrow was attempted by the construction
of artificial burrows, consisting of grooves
cut into the rock and covered with a glass
slide secured tightly in place with a plastic
adhesive. After establishing themselves in
such burrows, both P. dentigerum and P.
antillnrum were relatively quiescent, their
main activity consisting of the extension of
the introvert to the exterior. There was,
however, in the case of P. dentigerum, a
frequent peristaltic movement of the entire
body.
Movements of sipunculids, after removal
from their burrows, consisted not only of
extension and retraction of the introvert,
but also of a lengthening and shortening
of the trunk and, in the cases of P. antillarum and P. dentigerum, the contraction
and extension of the posterior extremity
from a flattened shape to a sharp point.
The posterior shield of Paraspidosiphon
steenstrupi can also assume a variety of
shapes, from flat to pointed.
Although not observed in the artificial
burrows, a rubbing movement of the body
surface against the wall of the burrow is
suggested by an examination of the cuticle
of large numbers of Lithacrosiphon
gurjanovae and Phascolosoma antillnrum.
In L. gurjanovae, various portions of the
trunk cuticle are frequently missing or in a
state of partial peeling. In approximately
10% of the P. antillarum examined, the
posterior extremity of the trunk was partially denuded of papillae (Fig. 5f). The
bare area was sometimes marked by small,
widely separated papillae, apparently in
the process of regeneration. Such a loss of
papillae might be explained by a frictional
movement of the posterior end of the
body against the blind end of the burrow.
Sipunculids, after removal from their
burrows, have never been observed under
laboratory conditions to bore into rock,
even though they have been exposed to
rocks for periods of 2 to 3 months in
aquaria. It seems probable, therefore, as
has been suggested previously (Sluiter,
811
1891), that a sipunculid remains in a single burrow throughout its lifetime, slowly
enlarging it as it grows. Relative to this
supposition, we have recently observed in
rearing sipunculid larvae in our laboratory
that juveniles of an unidentified species of
Aspidosiphon will crawl into the interstices of a calcareous rock and within a
short time establish themselves in small
holes, with anterior ends directed outward.
Three such animals have now been maintained for 5 months, during which time
they have remained in the same hole, the
opening of which has been observed to
increase gradually in diameter.
Possible Roles of Cuticular Structures and
Epidermal Glands in Boring
Because of the position of the animal in
the burrow with the introvert directed
toward the opening, it is unlikely that the
hooks and spines of the introvert play a
significant role in forming the burrow. Instead, they are probably utilized primarily
as food-getting devices. However, with the
continual movement of the introvert in
and out, undoubtedly the hooks and spines
rub against the anterior end of the burrow. If the rock is soft, this action might
break off particles of rock, resulting in
time in the enlargement of the opening of
the burrow. Similarly, the movement of
the introvert along the surface of the rock
might scour it and contribute to surface
pitting and gradual erosion.
The anterior shields, always directed
toward the mouth of the burrow, are most
probably opercular in function, sealing off
the opening of the burrow after withdrawal of the introvert. Further indication
that the shields serve as opercula rather
than as active boring organs is the frequent occurrence of considerable algal
growths on the anterior cones of L. gurjanovae. In contrast to the anterior shield, the
posterior shield is in a position, at the
blind end of the burrow, where it could
serve in the elongation of the burrow.
With its tough, thickened construction and
its ability to contract and expand, it could
812
MARY E. RICE
conceivably function in the mechanical
abrasion of the substrate.
The papillary protuberances and their
constituent cuticular platelets give a
rugged and tough consistency to the external body surface. This rough exterior, if
rubbed against the sides of the burrow,
could serve effectively in mechanical boring in friable rock. Hence, if frictional
movement within the burrow is assumed, it
seems plausible that both the posterior
shield and the cuticular papillae could be
utilized as boring structures in relatively
soft rock.
Sipunculids, however, are found not
only in soft rocks, but frequently they occur deeply burrowed in hard, compact
rock where few, if any, other organisms
penetrate. Their presence in hard calcareous substrates suggests the probability of a
chemical mechanism whereby the rock can
be dissolved or at least softened to the
point where mechanical processes might be
effective. No experimental evidence exists
for a mechanism by which chemical boring might be achieved in sipunculids. Attempts to demonstrate acidic substances in
the burrows of sipunculids or on the surface of the animals by means of small
pieces of pH paper have met with negative
results. Possible sources of a chemical
solvent or softener are the epidermal
glands which occur in great numbers over
the surface of the body and empty their
secretions to the exterior through the papillary pores. Little information is now
available on the nature of the secretory
products of the epidermal glands, but an
investigation of these secretions should be
considered in any future study of boring
mechanisms in sipunculids.
In conclusion, it is postulated that
sipunculid burrows form in calcareous
rock by chemical dissolution of the substrate and that a possible source of the
solvent may be the secretory products of
the epidermal glands. Furthermore, it is
presumed that the papillae and posterior
shield, if rubbed against the walls of the
burrow by movements of the body, could
contribute to the attrition of soft, friable
rock or of hard rock if the walls of the
burrow had been previously softened by
the solvent activity of secretory products.
REFRENCES
Gardiner, J. S. 1903. The iSfaldive and Laccadive
Groups, with notes on other coral formations in
the Indian Ocean. The Fauna and Geography of
the Maldive and Laccadive Archipelagoes 1:333341.
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