250 STUDIES IN GENERAL PHYSIOLOGY 
certain organs, but the specific reactions of an organ toward 
light and gravity are also dependent upon the nature of its 
substances. It might be believed, therefore, that the polyp- 
forming substances also determine positive heliotropism, 
while the root-forming substances determine negative heliot- 
ropism. These circumstances might therefore explain the 
apparent paradoxes in the reaction of Sertularia to light. I 
hope to be able to study this question experimentally, and 
therefore will not enter into any further theoretical dis- 
cussion. 
2. I mentioned in the previous volume of these studies 
that Bonnet and Dalyell had found that an organ of another 
kind may occasionally grow in place of one that has been 
lost. Dr. A. von Heider, of Gratz, called my attention to the 
fact that he, too, had observed and described such a case.! I 
will give his description in full here: 
I have often had the opportunity of testing in Cladocora the 
great powers of reproduction which Coelenterates in general are 
known to possess. Without discussing the rapid healing of wounds 
and the renewal of wornout portions of the body, the following case 
seems worthy of description. I cut off by a rapid incision, and as 
near the rim of the shell as possible, the polyp of a Cladocora, 
which was protruding a great distance beyond its shell, and 
allowed the animal to go on living in the aquarium. As early as 
the next day the tentacles of the animal, which had been robbed of 
its calcareous support, were entirely unfolded, the transverse 
wound at the opposite end had puckered to a conical scar, and the 
polyp moved over the bottom of the vessel by means of its ten- 
tacles. When examined with a lens some weeks later, the aboral 
end of the animal was completely healed and possessed of a plate 
running parallel to the oral plate, at the periphery of which were 
tiny elevations corresponding to the tentacles of the oral plate. In 
the course of two months these developed into full-grown ten- 
tacles. In the center of this new plate of tentacles was a round 
opening, the newly formed mouth, so that an entire oral plate had 
been formed at the cut end of the polyp, which differed in external 
1A, von HEIDER, Wiener Sitzungsberichte, Vol. LXXXIV, Part I (1881). 
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