THE SENSORY-NEUROMUSCULAR SYSTEM 317 



mission, which supposedly occurs by passage from cell to cell in 

 the general mass. The word "neuroid" is used in order to distin- 

 guish this process from neural transmission, which occurs in 

 animals having a nervous system. 



Ccelenterata. — The nervous system of the hydra (Fig. 129, 

 p. 267) illustrates a further development. The sensory cells and 

 the neuro-sensory cells are receptors, connected, directly in some 

 instances, with effectors in the form of the muscle processes of the 

 large ectoderm cells. Usuall}', however, there are nerve cells 

 between these sensory and muscular elements. The sponge has 

 only effectors, while hydra has a receptor-effector system, but lacks 

 the adjustors, which are found in higher animals. It is of interest 

 that coelenterates also possess independent effectors, in the form of 

 cnidoblast cells, since these seem to be stimulated directly and 

 therefore to function independently of the nervous system. The 

 merest beginning of centralization in the nervous system is seen 

 in the hydra in the concentration of nerve cells in a ring about the 

 hypostome at the bases of the tentacles and near the base of the 

 body (Fig. 128, p. 266). In some of the jellyfishes such cen- 

 tralized rings of cells are further developed. 



Annulata. — In segmented worms, like the earthworm, a central 

 nervous system is well developed, but the cellular mechanism 

 is comparable with that found in the hydra, with certain additions 

 (Fig. 144, p. 298). The sense-organs in the epidermis are com- 

 posed of groups of sensory cells, the receptors, from which processes 

 extend as nerve fibers running in nerves to the central system. 

 Here the processes may end in contact with the many finer 

 processes, or dendrites, of motor neurones which are comparaVjle 

 with the nerve cells of a hydra (cf. Fig. 129, p. 267), and from 

 which single processes, or axons, extend outward to the muscle 

 cells or effectors. There is, however, another tj^pe of nerve cell, 

 the dendrites and axons of which are restricted to the central 

 system. To these the term adjustor neurones may be applied. 

 In the longer reflex circuits such adjustor cells lie between the 

 processes of the sensorj' cells and the dendrites of efferent neurones 

 whose axons extend to the muscles. Communication from seg- 

 ment to segment is made possible by the processes of adjustor 

 cells that extend longitudinally in the nerve cord. The develop- 

 ment of the central system with its adjustor neurones constitutes 

 the important advance of the earthworm's nervous mechanism 



