116 THE KIDDLE OF THE UNIVEESE. 



of touch and an organ of movement. Both these 

 instruments are direct external projections of proto- 

 plasm ; the stimulus, which alights on the first, is 

 immediately conducted to the other by the psycho- 

 plasm of the unicellular body, and causes it to 

 contract. This phenomenon is particularly easy to 

 observe, and even produce experimentally, in many 

 of the stationary infusoria (for instance, the poterio- 

 dendron among the flagellata, and the vorticella among 

 the ciliata). The faintest stimulus that touches the 

 extremely sensitive hairs, or cilia, at the free end of 

 the cells, immediately causes a contraction of a thread- 

 like stalk at the other, fixed end. This phenomenon 

 is known asa " simple reflex arch." 



IV. — These phenomena of the unicellular organism 

 of the infusoria lead on to the interesting mechanism 

 of the neuro-muscular cells, which we find in the 

 multicellular body of many of the lower metazoa, 

 especially in the cnidaria (polyps and corals). Each 

 single neuro-muscular cell is a "unicellular reflex 

 organ"; it has on its surface a sensitive spot, and a 

 motor muscular fibre inside at the opposite end ; the 

 latter contracts as soon as the former is stimulated. 



V. — In other cnidaria, notably in the free swimming 

 medusaB — which are closely related to the stationary 

 polyps — the simple neuro-muscular cell becomes two 

 different cells, connected by a filament : an external 

 sense-cell (in the outer skin) and an internal muscular 

 cell (under the skin). In this bicellular reflex organ 

 the one cell is the rudimentary organ of sensation, 

 the other of movement ; the connecting bridge of the 

 psychoplasmic filament conducts the stimulus from 

 one to the other. 



VI. — The most important step in the gradual 



