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This is because they do not create a true gastrula during embryogenesis, and as a result do not produce a true endoderm or ectoderm.

We should reiterate here that the Porifera do not possess “true” tissues that are embryologically homologous to those of all other derived animal groups such as the insects and mammals. 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. As we have seen, the vast majority of invertebrate animals do not possess a defined bony vertebral endoskeleton, or a bony cranium.