Cnidaria structure
Different tissues but no organs yet:
Embryo ectoderm produces epidermis and nets of nerve cells (coordination of muscle contraction)
Embryo endoderm produces gastrodermis, layer of digestive cells
First gut cavity: evolution of extracellular digestion ? larger animals can be digested
Mesoglea: gelatinous material between epidermis and gastrodermis
Nematocysts: eject barbed “harpoon”; driven by osmotic pressure (high ion concentration inside of water-impermeable cell); can contain toxic proteins; food capture and defense