As a direct bending of the tube by mechanical force does not result 

 in this permanent change of form the author seeks to explain the 

 observed result by the suggestion that the animal, bending to one side, 

 reconstructs or adds to the walls of the tube on that side and so 

 forces it to maintain its new position. 



Again in a Serpula which lives in an inelastic calcareous tube, 

 experiment shows that by changing the direction of light the tube 

 becomes also changed in its direction. Here, however, the process is a 

 very slow one and results from the growth of the newly formed part of 

 the tube in the direction in which the worm turns its branchiae and 

 head under the influence of light ; the tube once formed does not beud. 

 The dependence of the arrangement of animal structures upon the 

 direction of light is shown again in the case of a certain sertularian 

 hydroid. When pieces of the stem are inserted upsidedown in sand 

 the old lower end, now exposed to the light, sends out both new stems 

 and new roots. The new stems, as they grow, take the direction of the 

 light rays and so do the new roots, but the former grow towards the 

 light the latter directly away from it; are then positively and nega- 

 tively heliotropic respectively. Adventitious roots coming out from 

 the inverted stem show again this same negative heliotropism. 



In the light of these and other experiments upon the direct response 

 of animals to stimulation by light, gravitation, contact, etc., the author 

 here protests against the introduction of " instinct " and " will " in the 

 explanation of such phenomena, relegating such expressions to the 

 same category as " vital force." 

 To return to the main paper. 



Another extension of heliotropic phenomena is made in the case of 

 the movement of the pigmental processes of the outer cell-layer of the 

 retina in man under the action of light. In this part of the subject 

 the author has no new observations to record, though his explanation 

 of the movements of orientations of eye and head as due to the above 

 heliotropism is sufficiently novel. The twelfth chapter concludes this 

 contribution to heliotropism. Here we find some new facts brought 

 together with some of those observed upon insects to show that the 

 movements of an animal when acted upon light depend upon its 

 morphological structure. Thus in a bilaterally symmetrical animal 

 the oral end is found to be more irritable than the aboral end, the 

 dorsal and the ventral sides not equally irritable, while symmetrical 

 points right and left of the median plane are equally irritable. Hence 

 arises the tendency to move directly towards or away from the light 



