41 6 Harry Beal Torrey. 



The rate of locomotion is slow. Half an inch in twenty-four 

 hours is a maximum rate. On a vertical surface the movement is 

 always directly upward. Gravity evidently determines the direction. 



The value of locomotion of this sort, and especially its negatively 

 geotropic character, would seem to lie in providing a means 

 whereby the hydroid may keep above the surface of the shifting 

 sands. 



The filaments of the hold-fast are also furnished with amoeboid 

 cells by which they are enabled to move out amongst the sand 

 grains to which they cling and anchor the stem {cf. amoeboid 

 movements of the tips of stolons of Campanularian hydroids). 

 One set of observations gave a rate of nine microns per minute at 

 the tip and five microns per minute halfway to the base of a fila- 

 ment several millimeters long. The free end is swollen and club- 

 shaped, with well developed ectoderm which not only provides 

 amoeboid cells but gland cells, which secrete the perisarc in which 

 the final strength of the filament as an anchor lies. The ecto- 

 derm of the remainder of each filament is attenuated almost to the 

 limit of visibility; the whole filament appears to be upon the 

 stretch, pulled out by the creeping club-shaped end. The endo- 

 derm of the filament is composed of a single column of cells such 

 as is characteristic of the endoderm of the tentacles of Campanu- 

 larian hydroids. These cells, under the tension, may become 

 much longer than broad, and retain these proportions whether the 

 filament is attached or free. A "setting" process seems to have 

 followed the stretching here, effecting the permanence of the 

 attenuation without cell division. 



The direction of locomotion of the filaments is always outwardy 

 but appears to be otherwise indeterminate. Arising below the 

 perisarc on the peripheral canals of the stem as solid outgrowths 

 with a deflection toward the proximal end, they creep along the 

 stem for a short distance, closely in contact after the manner of 

 stolons, and then push outward, secreting perisarc as they go. If 

 the stem is hung freely in the water, the filaments extend in all 

 directions. If it is in contact with the substratum, however, they 

 creep along the latter as soon as they come in contact with it. 

 If the substratum is sand, a filament pushes its way between the 



