marine life today. The first metazoans were a loose organization of individual
cells united for a common purpose, such as locomotion, feeding, and protection.
The most primitive metazoans probably comprised numerous cells, each with
its own flagellum. They grouped into a small, hollow sphere, and their flagella
beat the water in unison to propel the tiny animals through the sea.
From these metazoans evolved sedentary forms turned inside out and
attached to the ocean floor. Openings to the outside enabled the flagella now
on the inside to produced a flow of water, providing a crude circulatory sys-
tem for filtering food particles and ejecting wastes.These werethe forerunners
of the sponges (Fig. 31), the most primitive of metazoans.They existed in var-
ious shapes and sizes, some species possibly reaching 10 feet or more across, and
grew in thickets on the ocean floor. The body consisted of three weak tissue
layers whose cells could survive independently if separated from the main body.
The cells either reattached themselves or grew separately into mature sponges.
This ability results from the sponges’ lack of regulatorygenes that tell develop-
ing embryos where to put new body parts as is the case in complex animals.
Some sponges have an internal skeleton of rigid, interlocking spicules com-
posed of calcite or silica. One group had tiny, glassy spikes for spicules, which gave
the exterior a rough texture unlike their smoother relatives used in the bathtub.
Glass sponges consisted of glasslike fibers of silica. These hard skeletal structures
are generally the only sponge parts preserved as fossils. Sponges ranged from the
Precambrian to the present, but microfossils of sponge spicules did not become
abundant until the Cambrian.The great success of the sponges, which like other
animals extract silica directly from seawater to construct their skeletons, explains
why today’s oceans are largely depleted of this mineral.Figure 31The sponges
were the first giants of the
sea,growing 10 feet or
more across.
Historical Geology