FuRTHER INVESTIGATIONS ON HEtiotTROPISmM 101 
to their structure as radial organs. Such a Serpulida, if 
heliotropically irritable, must place the longitudinal axis of 
its cylindrical tube in the direction of the rays of light. If 
the caleareous tube is brought into any other position with 
reference to the source of light, the animal must make use 
of one of two possibilities in order to regain its proper orien- 
tation: either it must lengthen its calcareous tube and bend 
the newly growing part until the axis of the tube again 
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FIG. 12 
lies in the direction of the rays of light, or else leave its 
tube entirely and build a new one having the proper 
orientation. The animal makes use of the first of these 
possibilities. I experimented with Serpula uncinata. These 
Annelids inhabit calcareous tubes and are gregarious. 
Large white blocks are found in the Gulf of Naples which 
consist entirely of the tubes of countless numbers of such 
Annelids massed together. I noticed that the individual 
tubes in such a mass all had the same orientation, and in 
those cases in which the blocks showed the base upon which 
they had rested on the horizontal bottom of the ocean it was 
plainly visible that the tubes must have stood in the water 
with their longitudinal axes vertical. Serpula can, like 
Spirographis, move about freely within its tube. I laid a 
large block of innumerable annelid tubes, each of which 
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