42 BIOLOGICAL LECTURES. 



with a solid body plays the same role as did gravitation in the 

 case of Antennularia. In what way the contact may have an 

 influence shall be mentioned later. We may add, however, 

 one more point. In Antennularia, gravitation not only deter- 

 mines the place of origin of the various organs, but also the 

 direction of their growth ; the stem, growing upward, is nega- 

 tively geotropic, the root, growing downward, is positively 

 geotropic. In Pennaria, the nature of the contact not only 

 determines the place of origin of the various organs, but also 

 the direction of their growth. If we bring an outgrowing 

 polyp of Pennaria into contact with a solid body, the polyp 

 begins to grow away from the body, and the new stem is very 

 soon nearly perpendicular to the part of the surface with which 

 it came into contact. 



I have called this form of irritability Stereotropism. We 

 may speak of positive Stereotropism in the case of the root, 

 and of negative Stereotropism in the case of the polyp. 



Here, too, it may be asked whether contact with foreign 

 bodies, which in these experiments determined the arrange- 

 ment of the various organs, may not have the same effect in 

 the natural development of the organism. I believe that such 

 is the case. Negative Stereotropism forces the polyps to grow 

 away from the ground into the water, and so parts surroundetl 

 by water form i)olyps only. Positive Stereotropism forces roots 

 in contact with the ground to grow toward it, so parts in con- 

 tact with the ground give rise to roots onl)'. Thus it happens 

 that, under ordinary circumstances, in the animal we find roots 

 only at the base where it touches the ground. In other hydroids 

 the place of origin of the different organs is determined by 

 light, and in others we find more complicated relations. 



It may appear from the foregoing 



that such cases of heteromorphosis are 



confined to hydroids, but such is not 



the case. We find similar cases in 



TiDiicatcs. Ciomi iiitcstinalis (I'ig. 8), 



a solitary ascidian, has eye-spots around tlie two openings 



into the pharyngeal cavity, a and b. If we make an incision 



at c, eye-spots are formed on both sides of the incision. 



