The nematocysts of Microstoma are derived from Hydra upon which Microstoma feeds. These ingested nematocysts are delivered to the mesoderm by the endoderm of the flatworm. Within the mesoderm an amœboid cell takes charge of each nematocyst and as it is transported to the surface orients it so that the "sting-thread" has its discharging pole directed towards the exterior of the animal. Thus Microstoma has come to use the nematocysts of its prey as do the æolids use the nematocysts of their prey.