![]() ![]() The split between the Parazoa and the Eumetazoa (all animal clades above Parazoa) likely took place over a billion years ago. This clade currently includes only the phylum Placozoa (containing a single species, Trichoplax adhaerens), and the phylum Porifera, containing the more familiar sponges ( (Figure)). We will start our investigation with the simplest of all the invertebrates-animals sometimes classified within the clade Parazoa (“beside the animals”). However, one of the most ancestral groups of deuterostome invertebrates, the Echinodermata, do produce tiny skeletal “bones” called ossicles that make up a true endoskeleton, or internal skeleton, covered by an epidermis. Explain the various body forms and bodily functions of spongesĪs we have seen, the vast majority of invertebrate animals do not possess a defined bony vertebral endoskeleton, or a bony cranium.Describe the organizational features of the simplest multicellular organisms.Groups of cells in the nerve net form nerve cords that may be essential for more rapid transmission.By the end of this section, you will be able to do the following: The function of the nerve cells is to carry signals from sensory cells and to contractile cells. The nervous system is primitive, with nerve cells scattered across the body in a network. However, organs and organ systems are not present in this phylum. There are differentiated cell types in each tissue layer, such as nerve cells, enzyme-secreting cells, and nutrient-absorbing cells, as well as intercellular connections between the cells. Between these two layers is a non-living, jelly-like mesoglea. The outer layer is called the epidermis, whereas the inner layer is called the gastrodermis and lines the digestive cavity. ![]() All cnidarians have two tissue layers, with a jelly-like mesoglea between them.Īll cnidarians have two tissue layers. \): Cnidarians have two distinct body plans, the (a) medusa and the (b) polyp. ![]()
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