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THE BASAL METAZOANS: SPONGES AND CORALS 261
Until nearly the end of the Neoproterozoic, eggs hatching within the parent sponge, and
some 80% of geological time, the oceans of larvae released into the water.
the world were mainly occupied by rather There are over 10,000 species of sponge.
simple, usually unicellular, organisms. By the All are aquatic, and most are marine. Sponges
Ediacaran it was clear that this simple exis- are part of the sessile benthos, fi xed to the
tence was not enough, and more complex seabed, pumping large volumes of water – in
body plans were soon to develop their own extreme cases over 1000 L per day – through
ecosystems. Two groups, the Porifera and the their fixed but commonly fl exible bodies,
Cnidaria, form the basal parts of the meta- which act as filters for nutrients. The group
zoan tree, diverging during the Neoprotero- has a remarkable range of morphologies; the
zoic. Despite their origins in deep time and more specialized, stalked forms live in deep-
their relative simplicity, both maintained water environments and fl attened, dumpy
high diversity, notably as colonial organisms, forms prefer shallower-water, high-energy
throughout the Phanerozoic, frequently environments. Despite the apparent simplicity
becoming important parts of the planet’s reef of the sponges, the classification of the phylum
ecosystems. has recently undergone considerable revision
(Box 11.1). Some well-established calcifi ed
groups, such as the “chaetetids” and “sphinc-
tozoans”, are probably polyphyletic, merely
PORIFERA
representing convergence towards common
So he dissected sea sponges by night, grades of organization. The well-established
winter night after winter night . . . adult and diverse Demospongea, the common
and embryo human body parts by day, sponges, may too be polyphyletic. Despite
adult and larval sponge body parts by their relative simplicity, the complex relation-
night. ships of “sponge-grade” animals have yet to
be resolved.
Rebbeca Stott (2003) Darwin and the
Barnacle, on the sponge doctor, Robert
Grant
Morphology: examining a typical sponge
A typical sponge individual is not particularly
Most of us have used a bath sponge, probably complex or intellectually demanding to under-
a synthetic replica of the real thing. But ancient stand; it is nonetheless a remarkable organ-
peoples used sponge skeletons as an aid to ism. It is sac-shaped with a central cavity or
bathing and possibly exfoliation in some of paragaster, which opens externally at the top
the world’s earliest and most exclusive health through the osculum (Fig. 11.1). The sponge
farms. Most considered they were some form is densely perforated by ostia, small holes
of plant until proper biological study in the marking the entrances to minute canals
mid and late 1700s suggested they were through which pass the inhalant currents. In
animals – and at first they were classifi ed as simple terms, there are three main cell types:
corals. It was in fact Dr Robert Grant (1793– (i) flattened epithelial cells; (ii) collar cells or
1874), one time mentor to Charles Darwin, choanocytes, which occupy the internal cham-
who later established the Porifera as a unique bers and move water along by beating their
group in its own right. The poriferans or fl agella; and (iii) amoeboid cells, which have
sponges have a unique porous structure and digestive, reproductive and skeletal functions.
a body plan based at the cellular level of orga- Amoeboid cells can actually irreversibly
nization; they are said to lack true tissues. change into other cell types with other func-
Most lack symmetry, true differentiated tions. Nutrient-laden water is thus sucked
tissues, and organs, although their cells, like through the ostia, flagellated by the choano-
those of the protists, can switch function. cytes and processed by the amoeboids. Waste
They reproduce both asexually (by budding) products and spent water, together with repro-
and sexually with different cells expelling ductive products when in season, are ejected
clouds of eggs and sperm out through an upwards through the paragaster into the
opening; some are even viviparous, with the water column.
