2i8 THE BIOLOGY OF THE SEA-SHORE 



vertebrates, is composed of a number of comparatively 

 simple structural units, the nerve-cells or neurones, having 

 a typical arrangement. This includes (i) a sensory cell 

 extending from the body surface to the grey matter of 

 the spinal cord (with its body situated in the spinal 

 ganglion), along which impulses pass in to the central 

 nervous system ; (2) a motor nerve - cell, with its 

 body situated in the grey matter, connecting the spinal 

 cord with a muscle ; and (3) a connecting or internuncial 

 nerve- cell joining up the sensory and motor nerve- cells 

 within the central nervous system. This chain of related 

 parts, consisting of receptor, adjustor, and effector elements, 

 has been designated the neuro-muscular system (Parker, 1918), 

 since the term " nervous system " does not usually include the 

 muscles, or effectors, which are nevertheless an integral part 

 of the behaviour organisation. According to Parker {op. cit.), 

 this system probably did not originate as a nervous structure, 

 but the first trace of it is to be seen in independent effectors 

 such as the smooth muscle of the lowest multicellular 

 animals. ** This tissue, as seen in the oscular and pore 

 sphincters of sponges, represents muscle unassociated with 

 nerve and acting under direct stimulation from the environ- 

 ment. Such independent effectors are apparently open to 

 only a limited range of stimuli, particularly to those of a 

 physical type, and are relatively slow and sluggish in 

 response." To this simple effector system was next added 

 a receptive element in the shape of a sensory surface formed 

 by modification of epithelial cells in the neighbourhood 

 of the already differentiated muscle and particularly respon- 

 sive to pressure changes, chemical changes, and changes in 

 heat and light ; the disturbances set up by any of the above 

 stimuli being communicated to the muscles. In practically 

 every case of this kind the processes of the receptive cells 

 are applied not only to the muscle-cells, but branch to form a 

 plexus or " primitive nerve net " in such a way that a stimulus 

 affecting only a few of the sensory cells is conveyed to a 

 considerably greater number of muscle-cells. A system of 

 this kind is present in the tentacles of sea-anemones. More 



