90 SruprIEs IN GENERAL PHYSIOLOGY 
light rays determines the orientation of the organs of the 
plant. It is characteristic of the organs of sessile plants that 
heliotropic curvatures are produced when the plant is illu- 
minated from but one side. A growing stem continues to 
bend when illuminated from one side only until the growing 
tip lies in the direction of the rays of light. Progressive 
movement in the direction of the rays of light, which is the 
rule for free-moving animals and plants, is of course impos- 
sible for sessile organisms. Everyone who has cultivated 
flowers in a room has no doubt observed the heliotropic 
bendings in the plants. The question now arises whether 
these heliotropic curvatures can also be produced in sessile 
animals when illuminated from one side only. I shall show 
in the following pages that this is, indeed, the case. 
I 
1. The experiments described here were made on the large 
marine Annelid, Spirographis Spallanzanii. It lives in a tube 
which is quite flexible, yet sufficiently rigid to keep the ani- 
mal in a definite position. The tube is formed from the 
secretions of the animal. The aboral end of the tube is 
fastened (by a secretion) to stones or other solid objects. 
The gills of the animal, which are arranged at its anterior 
end in several spiral turns radial to the longitudinal axis of 
the animal, are usually found unfolded and projecting beyond 
the open end of the tube. As the tube is almost impervious 
to light, the latter will act chiefly upon the gills. So far as 
we know at present, the animal has no eyes. 
The animal can move freely inside the tube, the inner 
surface of which is perfectly smooth, and can be removed 
from it without the slightest injury by cutting open the tube. 
I have occasionally seen the worm leave the tube of its own 
accord, when the water in the aquarium became bad. 
The layman seeing these animals in the tubes with their 
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