STAR-FISH AND SEA-URCHINS. 283 
on two different aspects of the animal, the combined 
result of 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 
Kchinus, 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 showing 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. Secraped 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. Seraped 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- 
