272 University of California Publications. [Zoology 



six rows of axial cells, not placed with perfect regularity in lin- 

 ear series, yet the longest diameters of all horizontal, parallel 

 with each other and with the longest diameters of the corre- 

 sponding cells in the distal tentacles. In the full grown adult 

 there may be four rows of axial cells in the basal region of each 

 distal tentacle, ten in the greater part of the length of each 

 proximal tentacle. Both kinds of tentacles have narrow, fili- 

 form ends, where the cells retain their original arrangement in 

 a single row. 



The horizontal orientation of the elongated axial cells in the 

 adult bears a definite relation to the movements of the tentacles. 

 The latter move characteristically inward and outward (up and 

 down), never toward one side. The longest axis of each axial 

 cell is therefore always perpendicular to the plane in which its 

 tentacle moves. This condition is obviously well adapted me- 

 chanically to the direction of the movements. Since the latter 

 occur in the same planes from the time of the origin of the ten 

 tacles throughout the life of the individual, the inference is 

 readily suggested that the movements of the tentacles may actu- 

 ally determine the arrangement of the axial cells. Whatever 

 displacements of cells might be caused by the movements of a 

 tentacle would naturally take place in the plane of the move- 

 ments. 



It seems probable, therefore, that not only may the number 

 of axial cells in any cross section be conditioned by the size of 

 the tentacle at that point, but their arrangement may be con- 

 ditioned by the direction of the tentacular movements. 



VI. — Development of the Frustules. 



The root filaments or "frustules" (Allman) are produced 

 throughout the life of the polyp. They arise early in the devel- 

 opment, occasionally before any of the tentacles. At first there 

 is a tendency to scatter. They are rarely terminal, though they 

 are usually restricted to the proximal half of the stem. They 

 may appear singly, or by twos and threes, but only exceptionally 

 at the same level. They do not appear in response to a contact 

 stimulus, arising on all sides of a stem, whether it be resting on 



