In the monograph, for the first time in the world literature, on the basis of original information obtained using modern methods of morphological research, an analysis of the organization of mysterious groups of marine invertebrates, characterized by the absence of the nervous system and muscles, mouth and intestines, and unexpressed germ leaves, is given. These animals are the remains of the fauna of the oldest multicellular that inhabited the Earth in Precambrian time. Based on the analysis done, a new sub-kingdom Prometazoa is distinguished, uniting primitive multicellular. For zoologists, embryologists, paleontologists, marine biologists of a wide profile; can be used as a textbook on courses in zoology and comparative anatomy of invertebrates, embryology and paleontology.