Life Began in the

Invertebrates are not defined by any single common characteristic, but simply by not being vertebrates. Their heterogeneity is most notable in the ocean. Some 3.8 billion years ago, life arose in our planet's oceans. The species that inhabit the ocean waters show greater diversity than those found in other environments. Some forms of animal life, such as corals and sponges, are so simple that they are not able to move about on their own. Others, such as some cephalopods, show great intelligence and skill. •

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Penaeus kerathurus

AMERICAN LOBSTER

Homarus americanus

Penaeus kerathurus

AMERICAN LOBSTER

Homarus americanus

Euphausia superba

Unique Shapes

Some sea organisms, such as jellyfish, some sponges, and anemones, are very simple. Others, such as sea urchins and starfish (phylum Echinodermata), are more complex. However, all of these life-forms have a body plan called radial symmetry, which, in contrast to bilateral symmetry, is found only in the sea.

RADIAL SYMMETRY

RED STA

RFISH

Echinaster sepositus

SEA CUCUMBER

Thelenota sp.

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