COORDINATION 



199 



Even in some unicellular organisms certain portions of the proto- 

 plasm are especially differentiated for receiving and conducting 

 stimuli, and others for making effective such stimuli by contrac- 

 tions of the whole or parts of the cell. Indeed, Paramecium and 

 other Infusoria possess a neuromotor system which apparently 

 consists of a coordinating center, or motorium, from which con- 

 ductile paths extend through the cell to the cilia, etc. But 

 it is in the lower Metazoa, such as Hydra 

 and its allies, that we find the establishment »» ' <"A""* » — * 

 of definite nerve cells, some of which are 

 specialized for re- 



Fig. 136. — Diagram 

 of a simple type of recep- 

 tor-effector system found 

 in some Hydra-like ani- 

 mals. It comprises recep- 

 tors (6), or sense cells, 

 reaching to the body sur- 

 Hydra. The parallel face (a), with basal nerve 

 lines represent muscle net (c) connecting with 

 fibers. (After Schnei- muscle cells (d). (Slightly 

 der.) modified, after Parker.) 



Fig. 135. - - Nerve 

 cells in the ectoderm of 



Fig. 137. — Diagram of 

 a more complex type of 

 receptor-effector system, 

 found in some Hydra-like 

 animals. It comprises, in 

 addition to the receptor 

 (6) with nerve net (c) and 

 the muscle cells (e), an- 

 other nerve (ganglion) cell 

 (d) interpolated in the 

 nerve net. a, body surface. 

 (From Parker.) 



ceiving stimuli and others for conducting the excitation to cells 

 specialized for contracting (muscle cells), etc. Thus a simple 

 receptor-effector system arises which may be regarded as the 

 basis for the development of the elaborate neuromuscular 

 mechanism of higher forms, with receptors, or sense organs, 

 conductors, or nerves, and effectors, or muscles and glands. 

 Although from the functional point of view it is difficult to differ- 

 entiate the receptors, conductors, and effectors in the economy 

 of the organism, from the standpoint of anatomy the conductors 

 constitute a definite entity, the nervous system proper. (Figs. 135- 

 137, 144, 145.) 



The structural elements of the nervous system of all animals 

 consist of cells known as nerve cells, or neurons. In the lower 



