Final answer:
The three main tissue layers of flatworms are the ectoderm, mesoderm, and endoderm, which give rise to the skin and nervous system, muscle and connective tissues, and the lining of the digestive system, respectively. These layers are vital for the development of complex organ systems in these acoelomate invertebrates.
Step-by-step explanation:
The three main tissue layers of flatworms are ectoderm, mesoderm, and endoderm. These embryonic layers are critical because they give rise to all different types of tissues and organs within an adult flatworm. The ectoderm is the outermost layer that forms the epidermis and nervous system. The mesoderm is the middle layer giving rise to muscle cells, connective tissue, and other internal tissues such as the supportive mesenchymal cells which secrete materials like collagen and mucus.
Lastly, the endoderm is the innermost layer developing into the lining of the digestive system and other internal organs. Flatworms, as acoelomates, feature a solid body without a cavity between the epidermis and the digestive system. The presence of these three layers signifies a significant evolutionary step in invertebrates, allowing for the development of organ systems.