exist in the body of a protozoan and in fact there is experimental evidence to show that in certain infusorians the superficial protoplasm is somewhat differentiated as a receptive surface and that this protoplasm also serves as a conducting organ whereby, for instance, the activity of certain groups of specialized cilia in these animals is coordinated. These conditions, however, are found within the substance of a single cell and are so remote from those of a true nervous mechanism that, interesting and significant as they are, they had better be termed neuroid than nervous. They show at best that the protoplasm of the protozoan harbors operations that may develop in the multicellular animals into reflex processes 
Fig. 2. Diagram of the Canal System of a Calcareous Sponge (modified from Haeckel). The lateral Inlet pores receive water from the exterior, as shown by the arrows on the sides; the osculum at the apex discharges water to the exterior. rather than that the protozoans possess these processes, and that we must look among the simplest metazoans for the beginnings of a true neuromuscular mechanism.
In making a quest for the first stages in the development of the nervous system, it is important to keep in mind the relative significance of the three physiological elements already pointed out: the receptors, the adjusters and the effectors. A little reflection will show that these three are not likely to prove all of primary significance.
A receptor or sense organ alone would be of no service whatever to an animal; it would resemble a telephone receiver disconnected from the rest of the system. In a similar way the adjustor or central organ is useless without at least some other element in the reflex apparatus. The only mechanism sufficient in itself is the effector, which, if it can be brought into action by direct stimulation, may accomplish something serviceable to the animal. It is therefore improbable that we shall find multicellular animals that possess either receptors or adjusters without effectors, but it is conceivable that primitive metazoans may have effectors without other parts of the typical neuromuscular mechanism.
In a search for the earliest traces of the neuromuscular mechanism,
