. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. 6j2 "Biology of the Vertebrates IV. PRIMITIVE NERVOUS SYSTEMS It took a long time to develop the complex human nervous system. In a simple animal like an Amoeba, the whole body is a single cell-mass of proto- plasm, possessing a generalized capacity for irritability and contractility. Whenever a stimulus impinges at some spot on such a body there results a direct local response at the point of stimulation, but the news of it travels slowly to other pa
. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. 6j2 "Biology of the Vertebrates IV. PRIMITIVE NERVOUS SYSTEMS It took a long time to develop the complex human nervous system. In a simple animal like an Amoeba, the whole body is a single cell-mass of proto- plasm, possessing a generalized capacity for irritability and contractility. Whenever a stimulus impinges at some spot on such a body there results a direct local response at the point of stimulation, but the news of it travels slowly to other parts of the cell. The fact that an Amoeba in escaping from danger extrudes a pseudopod on the side opposite to the point of stimula- tion is an indication that some degree of protoplasmic conduction is present even here. It is out of the question in such a case to speak of sense organs or receptors, of communicating nerve fibers, adjusting centers or of reacting muscles, because they have not yet A B C Fig. 598. Diagrams of primitive nervous systems. A, an independent effector, a muscle cell such as occurs in the most primitive multicellular animals; b, simple receptor-effector system as found in tentacles of sea- anemones; c, more complex type of receptor-effector system, with the insertion of a so-called ganglion cell, as seen in various parts of the sea- anemone. (After Parker.) It has been suggested that the coelenterates show what may have been the first steps in the evolution of the neuromuscular mechanism. In Hydra, for example, the inner ends of some of the surface ectodermal cells develop muscular processes. When these cells are stimulated their inner portions contract without mediation of any sensory receptor. Parker has given the name independent effector to this type of cell which serves as both receptor and effector (Fig. 598). In most animals, including many coelenterates, these two functions are performed by separate cells. In the simplest arrangement
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