I Berliner geowiss. Abh. E 18 243 ~ 255 I: Berlin 1996 
Observations on Astraeospongium meniscum (Roemer, 1848) from the Silurian 
of western Tennessee: Constructional morphology and palaeobiology of the 
Astraeospongiidae (Calcarea, Heteractinellidae) 
Dorte Mehl & Joachim Reitner 
Mehl, D. & Reitner, J. (1996): Observations on Astraeospongium meniscum (Roemer, 1848) from the 
Silurian of western Tennessee: Constructional morphology and palaeobiology of the Astraeospongiidae 
(Calcarea, Heteractinellidae).- Berliner geowiss. Abh., E 18: 243-255; Berlin. 
Abstract: Astraeospongium is found to form secondary calcareous basal skeletons and thus may be 
attributed to the morphological grouping "coralline sponges". In A. meniscum, a bowl-shaped sponge with a 
massive skeleton of octactins, the ontogenetically youngest and still uncemented spicules are found at the 
upper, concave surface, particularly close to the edge. Towards the bottom, the spicules soon become 
altered through epitaxic cementation and recrystallization. From this skeletal architecture can be inferred 
that the soft tissues occupied only the uppermost portions of the basal skeleton. The skeletal structure is in 
many respects similar to that of the recent Petrobiona massiliana, although the ecological adaptations of the 
two species are fundamentally different. 
Kurzfassung: Astraeospongium bildet ein sekundäres kalkiges Basalskelett und kann somit der 
morphologischen Gruppierung "koralline Spongien" zugerechnet werden. In A. meniscum, ein schüsseI-
förmiger Schwamm mit einem massiven Skelett aus Octactinen, befinden sich die ontogenetisch jüngsten 
und noch unzementierten Spicula an der oberen, konkaven Seite, insbesondere nahe dem Rand. Zum Basis 
hin ändern sich bald die Spicula durch epitaktische Zementation und Rekristallisation. Aufgrund dieser 
Skelettarchitektur kann geschlossen werden, daß sich der Weichkörper nur in den obersten Bereichen des 
Skeletts befand. Die Skelettstruktur ist der der rezenten Petrobiona massiliana in vielerlei Hinsicht ähnlich, 
wenngleich sich die ökologischen Anpassungen der beiden Arten grundlegend unterscheidet. 
Authors addresses: Dr. Dorte Mehl, Institut für Paläontologie, Freie Universität Berlin, Malteserstraße 74-
100, D-12249 Berlin. Prof. Dr. Joachim Reitner, Institut und Museum für Geologie und Paläontologie, 
Universität Göttingen, Goldschmidtstr. 3, D-37077 Göttingen. 
Introduction 
The Heteractinellidae are an extinct group of 
Porifera belonging to the Calcarea, as stated by 
Reid (1967) and verified by Reitner (1992) on 
account of the mineralogy and diagenesis of their 
calcitic spicules. They were a long-living group of 
sponges, common in shallow reefal and platform 
environments throughout the Paleozoic. The 
earliest representative of the Heteractinellida in 
body preservation is Jawonya from the Middle 
Cambrian Daly Basin, Australia (Kruse 1987), and 
the latest is Wewokella from the Early Permian of 
Texas (Finks 1960).The taxon Heteractinellida 
currently comprises the Eiffelia-taxon (Rigby 1983) 
Octactinellidae Hinde, 1887 (= Astraeospongiidae 
Miller, 1889), and the Late Palaeozoic Wewokelli-
dae King, 1943, as defined by Rigby (1983). It is 
uncertain at this point, whether or not Jawonya can 
be attributed to the Wewokellidae. 
Some Heteractinellida, like Eiffelia, have 
skeletons composed of lose spicules only, whereas 
other representatives, such as Jawonya and 
Astraeospongium, are "coralline sponges" i.e. the 
spicules are integrated into a secondary basal 
skeleton. The Octactinellidae, to which the 
Astraeospongiidae belong, are characterized by 
octactins, which are bilaterally symmetrical 
spicules with a central, vertical ray and 6 
tangential rays equally distributed around spaced 
in mutual distances of ca. 60°. The central rays 
may be partly atrophied, especially in spicules 
located at the outer surface of the sponge. 
Astraeospongium menisc/..fffl is the most common 
representative of the Octa~ihellida 
Astraeospongium Roemer, 1854, the most 
common and best known genus within the 
Heteractinellidae, has been definitely documented 
from the Late Ordovician to Late Devonian, while 
isolated octactinellide spicules may indicate its ' 
presence in the Early-Middle Ordovician and Early 
Carboniferous as weil. It is certainly characteristic 
of the family Astraeospongiidae. Several species 
have been described from the Silurium and 
Devonian of USA and Canada, from the 
Ordovician to Devonian of the Baltic region, 
including erratic blocks found in northem Europe, 
and also from the Silurian and Devonian of Middle 
Europe, Scotland and Russia (Rietschel 1968, 
Rigby 1983). 
Material and methods 
The fossiliferous strata, in which these sponges 
were collected, are of Late Silurian age from 
localities described by Rauff in his major work 
1893-95. A large collection of Silurian sponges 
from western Tennessee have been kindly lent to 
D. Mehl by the Chicargo Field Museum. About 150 
weil preserved specimens of Astraeospongium 
meniscum in partly siliceous, partly calcitic 
preservation are included in the collection. 
According to Rigby (pers. comm.), Astraeo-
spongium is most common in the Late Silurian 
Brownsport Formation in western Tennessee, and 
this is most probably the locality of the specimens 
here investigated. Due to the correspondence in 
the fossil content with that of Late Silurian sponge-
bearing strata of New York, this series in 
Tennessee is often called "Niagara Group". It is a 
sequence of grey carbonates intercalated with thin 
marly fossiliferous beds. Siliceous concretions are 
common within these layers, and most organic 
remains are silicified as weil (Roemer 1860). The 
Porifera contained are mainly desma-carrying 
Demospongiae ("Lithistida"), but also Astraeo-
spongium and some stromatoporoid demosponges 
are common. 
Most specimens are completely silicified 
and their large star-shaped spicules, weil visible on 
the outer surface, have normally become internally 
obscured due to recrystallization. Measuring of 
spicule sizes was carried out on the most 
completely preserved silicified specimens. A few 
specimens in calcitic preservation were processed 
into thin sections and studied in light microseope. 
The distribution of spicules throughout the entire 
sponge was investigated, as weil as the 
microstructure of single spicules in polarized light 
and also by UV-microscopy. The specimens will be 
returned to the Chicargo Field Museum. 
Systematics and nomenclature of the 
Heteractinellidae and Astraeospongium 
The taxonomie name "Heteractinida" was 
introduced by Laubenfels (1955) to include the 
families Chancelloriidae, Astraeospongiidae, and 
Asteractinellidae. This grouping is definitely non-
monophylitic, since Chancelloriidae do not belong 
to the Calcarea, and may not be sponges at all 
(Bengtson & Missarzsky, 1981). We here prefer 
the use of the original taxonomie name 
Heteractinellida Hinde (1888), as recommended 
by Rietschel (1968). 
The genus Astraeospongium Roemer,1854 
was based on the type species Astraeospongium 
meniscum,first described as Blumenbachium 
meniscum Roemer, 1848. In his later publication, 
Roemer (1860) described the species as 
Astraeospongia meniscus and listed Blumen-
bachium meniscus and Astraeospongium menis-
cus as younger synonyms. But in Roemer (1860) 
244 
there is no indication of any argument or intention 
to change original name. As correctly stated by 
Rietschel (1968), such a correction would be 
invalid according to the international rules of 
nomenclature, but in this case, a correction has 
not even been introduced, because no evidence of 
an emendation of intention of changing the name 
can be recognized in Roemer (1860). In the 
literature, both names have been used (e.g. 
Reitner 1992), but Astraeospongia is an invalid 
name! The correct name of this genus and type 
species is Astraeospongiqm meniscum. 
Morphology and spicule organization of 
Astraeospongium meniscum 
Astraeospongium meniscum is thick-walled, bowl 
to saucer shaped with a skeleton composed of 
large, star-like octactins. The size of the sponges 
varies between 3 and 9 cm, most of the specimens 
being 3.5-5 cm in diameter. Largest are the 
spicules found at the slightly concave surface, 
which in reconstructions is generally considered to 
be the upper side of the sponge. The sponge body 
is very variable from flat dish-like to conical, 
almost cup-shaped, but the central depression 
(paragaster) is normally very shallow (PI. 1, Figs. 
1-2; PI. 2, Fig. 1). Generally at the concave 
surface, the large spicules with their six 
paratangential rays parallel to the surface and the 
distal rays atrophied look most clearly star-shaped. 
They appear to be without any special orientation, 
often rays of neighbouring spicules overlap or 
show various patterns of overlapping and 
interfingering (PI. 2, Figs. 3, 5; PI. 3, Fig. 1; PI. 4). 
Comparing the concave (assumed) top with the 
convex (assumed) bottom, it can be seen that 
there are no signs of attachment to be found and 
that the spicules of the bottom~ are considerably 
smaller and less distinctive than the pronounced 
star-shaped ones of the top (PI. 1, Figs 3-6). At the 
convex side of most specimens, the limits between 
the spicules are more or less obscured, and it is 
hard to distinguish or measure single spicules. 
Further, the largest spicules are always seen in the 
central area of the conve>G side, whereas sm aller 
ones are found on eigther side of the sponge body, 
but mainly close to the edge. In some of the 
silicified specimens, especial ly the larger spicules 
exhibit central cavities running through the rays 
similar to weathered out central canals (PI. 2, Fig. 
2; PI. 3, Fig. 2). 
Most specimens in the collection are 
silicified through and through, and they are hollow 
or internally completely recrystallized with all 
originall structures destroyed.Thin sections were 
made through some of the few specimens in 
calcitic preservation, which still reveal relics of 
internal features (PI. 2, Fig. 3-6) The thin sections 
show single spicules fairly weil distinguishable in 
the top region of the sponge body, especially close 
to the edge. 
About 1-2 mm towards the base, the 
spicules become obscured, successively 
recrystallized and fused together by cements (PI. 
2, fig. 3). In the middle of the sponge, the spicules 
are fused into a massiv calcareous basal skeleton 
without any open spaces or channels. When 
viewed under the light microscope, single spicules 
from the upper, marginal area show a central core, 
which behaves as a single crystal of calcite, giving 
unit extinction under crossed nicols. 
On the five best preserved specimens, 
measurements of total ray sizes in spicules were 
carried out if possible · at the inner and outer 
surfaces to get a significant number of 
measurements (according to the .preservation). 
Spicule ray lengths were measured from centers 
of spicules to ray ends. Thicknesses of rays were 
measured at the middle of the rays. Following 
mean values were found: 
Mean Size (mm) Length Thick- Thickn.l 
ness Length 
S~. nO.1 
Inner center 3.67 0.70 0.19 
Inner edge 2.62 0.28 0.12 
Outer edge 1.80 1.19 0.11 
S~.no.2 
Inner center 3.00 0.52 0.18 
Outer center 1.88 0.65 0.36 
S~.no.3 
Inner center 3.16 0.69 0.22 
Inner edge 1.41 0.14 0.09 
Outer center 2.39 0.58 0.25 
Outer edge 0.85 0.08 0.10 
S~.no. 5 
Inner center 2.44 0.46 0.19 
Outer edge 1.88 0.16 0.09 
S~. no.14 
Inner center 1.98 0.43 0.22 
Outer edge 1.35 0.15 0.11 
245 
Text-figure 1: Spicule arrangement in 
a weil preserved heteractinellid 
sponge from the Eifelian Freilingen 
Formation (Middle Devonian, Rom-
mersheim near Prüm/Eifel, Germany) 
with strong affinity to Astraeo-
spongium with respect to the spicular 
skeleton, but the spicules appear less 
densely cemented. Also, the sponge 
body as weil as the spicules are 
considerably smaller alltogether, and 
the sponge body is irregular to platy in 
stead of saucer shaped. These 
sponges might be attributed to 
Magenia davidi Finks 1995 from the 
Silurian (Niagara) Henryhouse 
Formation of Oklahoma, a genus so 
far unknown in Europe, Scale 1 mm. 
Obviously, the size of individual spicules, 
between different specimens as weil as within the 
same sponge, is very variable. However, mainly 
the total size of spicules seems to vary throughout 
the sponges. The relationships between spicule 
thickness and length show relatively constant 
values, depending only on the position within the 
sponge body. Here three main regions can be 
distinguished: 
Inner central region: 
Outer central region: 
Inner and outer side close to edge: 
0.18-0.22 
0.25-0.35 
0.09-0.12 
Thus follows that the spicules of the outer 
basal region are rather small and stout with ray 
thicknesses up to 1/3 of the length. In the inner 
central region, where mainly the largest spicules, > 
5 mm in diameter, are found, the relationship 
between thickness and length is about 1/5. 
Interestingly, close to the edge on both inner and 
outer surface the smallest spicules occur, some of 
which measure only 0.85 min in length and 0.07 
mm in thickness of the rays; In this area, spicules 
are compareably thin, thickness of rays only about 
1/10 of total ray length. These local differences in 
spiculation are obvious and 'can be seen by close 
observation of the sponge in a light m icroscope , or 
using a good hand lense (see PI. 3). 
Structure and function of the entire skeleton of 
Petrobiona massiliana and Astraeospongium 
mensicum 
The spicules of Astraeospongium are characte-
rized by an optically uniform core of surrounded by 
a thick shale that shows diffuse extinction patterns 
. under crossed polars. This indicates that the 
spicules were formed in two phases by diffent 
processes: In the first place, the spicules were 
secreted as single crystals of calcite, secondly 
calcitic cements precipitated around the initial 
spicules and multiplied their thickness. Similar 
processes have been documented and described 
in detail by Reitner (1989a) on the formation of 
basal skeletons in the recent pharetronid sponge 
Petrobiona massiliana. 
P. massiliana (PI. 4, Figs. 1-4) is the only 
modem pharetronid sponge with a true basal 
skeleton. The basal skeleton is formed by 
aggregates of elongated units and flat crystal 
structures (cement chips). The elongated units are 
in vivo and early diagenetically altered spicules 
often enlarged by epitaxial crystal growth (Reitner 
1989a, 1992). The flat crystal bodies (cement 
chips) grow in spaces made by the exopinacoderm 
when growing sideward and formed the rigid basal 
skeleton frame. In old portions of the basal 
skeleton, the monocystalline spicules are extremly 
altered into polycrystalline spherulites, prismatic, 
and granular structures. The stable isotope 
composition of the basal skeleton differs only little 
from that of normal marine water, and the 
formation is controlled mainly by kinetic effects. 
Vital effects, such as high amounts of respirative 
C02, may be responsible for the slightly lighter 
values than expected when formed only under 
equilibrium conditions (Reitner 1989a). The mean 
values are 813C +2,1 and 8180 +1. 
TM living tissue of the sponges is restricted 
to the top area of the massive basal skeleton. The 
tissue is thin and charaterized by large choanocyte 
chambers « 1 00 ~m) . The mesohyle is weakly 
developed and only few symbiontic bacteria are 
present. Within the choanocyte chambers, bacteria 
are often filtered and trapped by the microvilli . The 
tissue also exhibits cryptic areas which are located 
in canals within the basal skeleton (Vacelet 1990, 
Reitner 1992). Within these canals, resting cells 
are enriched which probably allow to recover dead 
portions of the basal skeleton. The unique basal 
skeleton has probably the function to protect the 
crypt cells. Comparable strategies are realized by 
the chaetetid demosponge Spirasfrella (Acantho-
chaetetes) wellsi and the poecilosclerid demo-
sponge Merlia normani (Vacelet 1990, Reitner 
1989b, 1992). The basal skeleton of P. massi/iana 
is a modified chaetetid-type basal skeleton and 
could be described as a large modified gemmula 
cyst comparable with so-called ·aporate 
sphinctozoans· (Celyphia sp., see Reitner 1992). 
The entire skeletal structure of Petrobiona is 
to some degree comparable to the extinct 
heteractinellid calcareous sponge Astraeo-
spongium. This sponge is meniscus-shaped and 
formed by layers of large flat regular octactins. 
The central part of the skeleton is made of 
epitactically enlarged spicules, which close the 
major parts of the open space. Only the upper 
portions of the sponge exhibit open spaces, which 
indicate that in this area the living tissue was 
located. 
246 
. Neither in other recent nor in fossil sponges, 
have we observed a strong neomorphism as seen 
in Petrobiona linked with an alteration into 
polycrystalline structures. It is very probable that 
the strategy of Astraeospongium was in some 
aspects similar to that of Petrobiona. However, 
there are some significant differences: Petrobiona 
is restricted to firm substrates and lives in cryptic, 
dark niches, These environments are normally 
characterized by poor nutrient supply and some 
other changes in water quality. These conditions 
may explain the unique surviving strategy through 
resting cells. Astraeospongium was probably 
adapted to stable soft bottoms and has never been 
found in former cryptic niches. The meniscus 
shape of the basal skeleton reflects the adaptation 
to soft bottoms. The formation of the basal 
skeleton by early diagenetically altered spicules is 
comparable with that of Petrobiona. However, no 
traces or remains of intemal canals which might 
have contained resting cells were observed. It is of 
course possible that such narrow canals were 
secondarily closed through diagenetical 
cementation. The closely related Astraeoconus 
calcarius Rietschel, 1968 is described to have weil 
defined pores between the spicules. Also new 
species from the Devonian Eifel (Germany), which 
is weil attributable to the Astraeospongiidae, 
appears to be have a skeleton of lose, or hardly 
cemented spicules with cavities inbetween (Text-
Fig. 1). So there may weil be variations in the 
intensity of cementation, and the inclination to 
build basal skeletons within the Astraeospongiidae. 
However if there were originally no canals in the 
basal skeleton, we must assume that Astraeo-
spongium did not store any resting cells. This 
hypothesis corresponds with the adaptation to 
open marine shallow water conditions, where, 
contrary to cryptic niches, food supply is normally 
not the main limiting factor. The survival strategy 
by storing resting cells is a special method of long 
term survival in the extreme environments of 
cryptic niches. 
Acknowlegdements ) , 
D. Mehl thanks the Chicargo 'Field Museum for the 
generous loan of a large collection of sponges 
from the Silurian of TenneSsee, which contains, 
among others, the specimens here studied. Mr. R. 
Leunißen (Kreuzau) is thanked for sending 
heteractinellid sponges from the Eifel, and Mr. U. 
von Hacht (Hamburg) for interesting discussions. 
The Deutsche Forschungsgemeinschaft (DFG) is 
acknowledged for financing research on the 
Porifera (Me 1151-212). 
References 
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Coeloscleritophora - a major group of 
enigmatic Cambrian metazoans.- U.S. geol. 
Surv. Open-File Rep., 81-743: 19-21 
Finks, RM. (1960): Late Paleozoic sponges from 
the Texas region. The siliceous sponges. 
Bull. Amer. Mus. Nat. Hist, 120: 160 pp 
Hinde, G. J. (1887-1888): A monograph of British 
fossil sponges, 1. Sponges of the 
Palaeozoic and Jurassic strata.- 1-92 
London (1887), 93-188 London (1888) 
Laubenfels, M. W. de (1955): Porifera.- In: Moore, 
R C. (edit.): Treatise of Invertebrate Pale-
ontology, E: 21-112 
Rauft, H. (1893): Paleospongiologie.- Palaeonto-
graphica, 40: 1-232. 
Rauft, H. (1894): Paleospongiologie.- Palaeonto-
graphica, 41: 233-346. 
Rauft, H. (1895): Paleospongiologie.- Palaeonto-
graphica, 43: 223-272. 
Reid, RE.H. (1967): Hya/ostelia Zittel and the 
Heteraetinellida.- Geol. Mag., 104: 398-399 
Reitner, J. (1989a): Struktur und Diagenese der 
Basalskslette bei rezenten Pharetroniden 
unter besonderer Berücksichtigung von 
Petrobiona massiliana Vacelet & Levi 1958 
(Minchinellida, Porifera).- Berliner geowiss. 
Abh., (A), 106: 343-383; Berlin 
Reitner, J. (1989b): Lower and Mid-Cretaceous 
coralline sponge communities of the boreal 
and Tethyan realms in comparison with the 
modem ones.- In: Wiedmann, J. (ed.) Creta-
cous of the Western Tethys. Proc.3id Inter-
nat. Cretaceous Symp. 851-878, Tübingen 
1987; Schweizerbart (Stuttgart) 
Reitner, J. (1992): .Coralline Spongien". Der Ver-
such einer phylogenetisch-taxonomischen 
247 
Analyse . .., Berliner geowiss. Abh.(E), 1: 1-
352 S ! 
Rietschel, S. (1968): Die Oetaetinellida und ihnen 
verwandte paläozoische Kalkschwämme 
(Po rife ra , Calcarea).- Paläont. Z., 42 (1/2): 
13-3~ 
Rigby, J. K. (1983): Heteraetinida.- In: Rigby, J. K. 
and Steam, C. W. "Sponges and 
Spongiomorphs. Notes for a short course".-
Univ. Tennessee Dep. Geol. Sci. Stud. 
Geol., 7: 70-89. 
Roemer, F. (1848): Über eine neue Art der Gat-
tung Blumenbachium (König) und mehrere 
unzweifelhafte Spongien in obersilurischen 
Kalkschichten der Grafschaft Decatur im 
Staate Tennessee in Nord-Amerika.- N. Jb. 
Miner. Geogn. Geol. Petrefakten kunde, 
1848: 680-686, Stuttgart. 
Roemer, F. (1854): Palaeo-Lethaea, Kohlen-
Periode.- In: Bronn, H.G. & Roemer, F. 
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Stuttgart. 
Roemer, F. (1860) Die silurische Fauna des 
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Trewendt-Verlag, Breslau. 
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248 
Plate 1 
Astraeospongium meniscum 
Figs. 1-2: Specimen 9, view of the upper concave surface and lateral view, scale 5 mm. 
Figs.3-4: Specimen 12, upper surface shows large star-shaped interfingering spicules and convex 
bottom surface where the spicules are smaller and strongly cemented, scale 5 mm. 
Figs 5-6: Specimen 26, star-shaped spicules of the weathered upper surface exhibit pseudo central 
canals, and the heavily cemented bottom shows no more open spaces between spicules, scale 
5mm. 
249 
250 
Plate 2 
Fig. 1: Astraeospongium meniscum (spec. 26) seen trom the concave surface with the interfingering 
pattern ot star-shaped spicules and pseudo central chanals weil visible, scale 5 mm. 
Fig. 2: Focus view ot single spicules trom the surface with pseudo central canals which result trom 
weathering out ot the primary co res within the calcitic spicules, scale 1 mm. 
Fig. 3: Vertical thin section through Astraeospongium meniscum (spec. 32) in polarized light. The 
entire sponge is massive built up by calcitic spicules embedded in the secondary basal 
skeleton. In the uppermost part, interpreted as the ontogenetically youngest, the spicules are 
still only losely attached . Successively towards the base, they are cemented into a dense basal 
skeleton, scale 1 mm. 
Figs. 4-6: Focus on single spicules in polarized light shows the central core as a single crystal giving unit 
extinction under crossed polars, all scales 100 \-Im. 
251 
252 
Plate 3 
Astraeospongium meniscum, details of spicules at the upper concave surface. 
Fig.1: 
Fig.2: 
Fig.3: 
Fig.4: 
(spec. 14) outer margin of the concave surface with the spicules loosely attached, most 
spicules are relatively thin rayed, since the secondarily cemented basal skeleton has not 
become massive yet. This area is interpreted as the ontogenetically youngest part of the 
sponge, there are still large open spaces (now filled with sediment) between single spicules. 
Central area of concave surface (spec. 26) shows very large spicules with pseudo central 
canals from weathered out primary spicule cores. Secondary cementation of basal skeleton is 
already weil developed, and there is practically no room lett between ·single spicules. 
Close to the margin (spec. 5) of concave surface, Most spicules are rather thick rayed, although 
generally smaller than those of the central area. 
Outer marginal area (spec. 23) of concave surface, where the recently secreted, youngest 
spicules are found . The spicules here are generally small and thin rayed, since the cementation 
of the secondary calcitic skeleton is not yet very advanced. 
All scales 1 mm 
253 
Plate 4 
Figs.1-4: 
Fig.1: 
Fig.2: 
Fig.3: 
Fig.4: 
Fig.5: 
254 
Petrobiona massiliana Vacelet & Levi 1958 (Minchinellida, Calcaronea) 
Vertical section of the youngest portion of P.massiliana. The top area is characterized by a 
prominent cover of modified triaenes and calthrops calcitic spicules. The basal skeleton is 
formed by older spicules modified by an intense in vivo neomorphism and epitaxial crystal 
growth. The areas inbetween are filled with living tissue. The actively living tissue is restricted 
to the uppermost portion. Resting cells are located within canals inside the massive basal 
skeleton, scale 1 00 ~m. 
Spicule arrangement of the dermal layer with ostia. Critical point dried specimen . SEM, scale 
50 ~m . 
Horizontal section through the dermal area, scale 1 00 ~m . 
Boundary layer between the basal living tissue and the basal skeleton. Entrapped spicules and 
already altered spicules are present, SEM, scale 25~m . 
Astreaspongium meniscum, upper surface. The dermal area exhibits a comparable spicule 
pattern as that seen in Petrobiona. This type of heteractinid sponge exhibits a basal skeleton 
formed by modified epitaxially enlarged octactine spicules. Only the upper part of the sponge 
was occupied by living tissue as observed in Petrobiona. Traces of resting cells canals were not 
observed, scale 5 mm.