. The elementary nervous system. Nervous system; Nervous system -- Coelenterata. 202 THE ELEMENTAEY NERVOUS SYSTEM have other functions than the control of muscles, it ap- pears that neither of these names is especially appro- priate. They are the cells in the simpler types of nerv- ous system from which the neurones of the more complex types have been derived. Hence the name protoneurone, as already suggested, is not inappropriate (Parker, 1918 a). A receptor-effector system whose nerve-net in- cludes protoneurones is, therefore, a step nearer a cen- tralized nervous system of the higher anim
. The elementary nervous system. Nervous system; Nervous system -- Coelenterata. 202 THE ELEMENTAEY NERVOUS SYSTEM have other functions than the control of muscles, it ap- pears that neither of these names is especially appro- priate. They are the cells in the simpler types of nerv- ous system from which the neurones of the more complex types have been derived. Hence the name protoneurone, as already suggested, is not inappropriate (Parker, 1918 a). A receptor-effector system whose nerve-net in- cludes protoneurones is, therefore, a step nearer a cen- tralized nervous system of the higher animals than one in which the protoneurone is not present. Although a receptor-effector system may consist of highly differentiated sense organs connected by a protoneurone nerve-net with distantly located muscles, it nevertheless is very far from representing even a simple central nervous system. First of all, such a receptor- effector system exhibits in a marked degree a state of diffuse- ness. The animals possessing such systems have no single nervous organ to which their nervous experience may be said to be referred and from which their im- pulses to response emanate. Their nervous activities are uncentralized. Each important organ, like the tentacle or the foot of an actinian or the column of Corymorplia, has within itself its own neuromuscular organization, and the autonomy thus conferred on this part is one of the characteristics of the effector-receptor system. This system is further characterized by great diffuse- ness of transmission. Any nervous impulse started up. m Fio. 47.—Diagram of a com- plex type of receptor-effector sys- tem such as is seen in many parts of sea-anemones. It consists not only of receptors r, with their nerve-nets, and of muscle cells m, but also of the so-called ganglion cells g in the Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance
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