Primitive gut

... the most common and familiar forms) Description Sponges are the most primitive of the multicellular invertebrates. They lack a mouth and gut. The bodies are organized around a system of water canals. Sponges feed, breath, reproduce, ...

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... and mesoderm --> ectoderm = skin for example --> endoderm = primitive gut (the lungs are formed out of this structure) --> mesoderm is divided into four pieces: ...

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