This chapter describes the evolution of invertebrate animals such as arthropod insects and mollusks. Approximately 575 Ma, strange frondlike organisms began to appear in the seas in the Avalon Explosion. Sea anemones evolved within this group, which then evolved into acoelomates (flatworms) and pseudocoelomates (rotifers). Coelomates such as arthropods and mollusks began to rapidly evolve and diversify in the Cambrian Explosion 541 Ma, and nearly all coelomate phyla appeared in the fossil record by 518 Ma.
The animal kingdom began with the evolution of stationary frond like organisms in the sea. There were several different types of organisms and thousands of fossils on the Avalon Peninsula in Canada (575-560 Ma); thus, scientists call this period the Avalon Explosion. Sea anemones evolved within this group by 560 Ma.
Cnidarian sea anemones are radially symmetric and only have two tissue layers (diploblastic), endoderm and ectoderm. Most moving animals have a triploblastic bilaterian body plan. Triploblastic refers to three layers: ectoderm, mesoderm, and endoderm. Bilaterian refers to the construction of animals along two axes, front to back and top to bottom, with symmetry between the left and right sides of the animal.
The fossil record of the Late Ediacaran and early Cambrian shows a gradual evolution of animal complexity from 560 to 541 Ma (Figure 8‑13), beginning with cnidarian sea anemones. In addition, genetics and physiology link the moving animals to cnidarian sea anemones. During the period from 540 to 520, the appearance of early representatives of invertebrate phyla shows a gradual evolution of the invertebrates.
Genes, nerves, and cell types link cnidarian sea anemones to invertebrate annelid worms, arthropods (insects), and mollusks. Sea anemones have a nerve net, muscles, and body plan genes, which were the origin of the nervous and muscle systems of moving animals, as well as their body patterning genes.
In general, mollusks are more appealing than other invertebrates. Instead of slithering or crawling around, they live in cute little shells. The mollusks include the gastropods (snails), pelecypoda (clams), and cephalopods (squids and octopuses). The study of mollusks demonstrates the relationship between physiology and lifestyle. For example, many do not need efficient circulatory systems because they are slow moving.
The arthopod phylum has the largest number of species and the largest population of animals of all the animal phyla (Figure 8‑27). Some of the major groups of arthropods are crustaceans, spiders, and insects.
The deuterostomes include three phyla: chordates (includes vertebrates), echinoderms (starfish and sea urchins), and hemichordates (strange little animals). The chordate phylum includes three existing subphyla: urochordates (tunicates or sea squirts), cephalochordates, and vertebrates. Scientists know that we are more closely related these invertebrates than other invertebrates because of a similar embryogenesis pattern.
After the change in position of the sun and moon at the end of the fourth age, Moses described the natural origin of animals in the sea and flying creatures on land at the beginning of the fifth age.