INVERTEBRATE ZOOLOGY IN THREE BOOKS - BOOK 1 - H.Y. Shcherbak - 1995

SUBKINGDOM MULTICELLULAR ANIMALS (METAZOA)

SECTION PRIMITIVE MULTICELLULAR ANIMALS (PROMETAZOA)

This section includes primitive Multicellular animals whose body is formed by a primary external flagellated or ciliated Cell layer—the kinoblast—and an internal layer containing amoeboid, supportive, contractile, skeletogenic, and generative elements—the phagocytoblast. True germ layers (ectoderm, endoderm, and mesoderm) are absent, and they lack a Mouth, gut, Nervous system, and Muscles.

Primitive multicellular animals are microphagous, meaning they feed on fine particulate matter (Bacteria, unicellular Algae, detritus particles) via phagocytosis or pinocytosis. Their mode of Nutrition is associated with individual cellular activity. Consequently, in terms of their feeding mechanism, Prometazoa do not differ from Protozoans.

Locomotion in these multicellular animals is achieved not by muscles, as in true Multicellular Organisms, but by means of flagella or cilia, much like in protozoans. Similar to Protozoa, they exhibit irritability; however, this is a property of Cell Cytoplasm rather than a nervous system, as seen in true multicellular animals. The primitive multicellular animals comprise four extant phyla: Placozoa, Orthonectida, Dicyemida, and Sponges (Porifera).