This clade currently includes only the phylum Placozoa (containing a single species, Trichoplax adhaerens), and the phylum Porifera, containing the more familiar sponges ( Figure 28.2). 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.By the end of this section, you will be able to do the following:
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