I-YRTHER INVESTIGATIONS ON HELIOTROIM s \i lot 



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 calcareous tube is brought into any other position with 

 reference to tho 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 iicicli/ (jrotriinj \><ir\ until the axis of the tube again 



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 011 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 

 largo block of innumerable annelid tubes, each of which 



