STAR-FISH AND SEA-URCHINS. 283 



on two (lifrerent aspects of the animal, the combined 

 result uf these two stimuli is that of furnishing a 

 very pretty instance in physiology of the physical 

 principle of the parallelogram of forces. Thus, for 

 instance, if two stimuli of equal intensity be applied 

 simultaneously at the opposite sides of a globular 

 Echinus, the animal begins to walk in a direction 

 at right angles to an imaginary line joining these 

 two points. And, generally, wherever the two 

 points of simultaneous stimulation may be situated, 

 the direction of the animal's advance is the 

 diagonal between them. As showino- in more detail 

 how very delicate is the physiological balancing of 

 stimuli which may be produced in these organisms, 

 and consequently the manner in which we are able 

 to play, as it were, upon their geometrically 

 disposed nervous systems in illustration of the 

 mechanical principle of the composition of forces, 

 I shall quote a series of observations. 



" 1. Scraped with a scalpel the equator of an 

 Echinus at two points opposite to each other — 

 animal crawled at right angles to the line of injury. 



"2. Similarly scraped at the ab-oral pole — no 

 effect. There was no reason why injury here should 

 determine escape in one direction rather than in 

 another. 



" 3. Scraped similarly near the oral pole, and 

 half-way between pole and equator — little or no 

 effect. 



"4. Scraped in rapid succession five equatorial and 

 equidistant injuries — Echinus crawled actively in 

 one determinate direction; the equal and equi- 



