Biological Studies on Corymorpha. 419 



the hydranth. The perisarc is hardly sturdy enough at any time 

 to afford any support to the stem. Its adhesive character, how- 

 ever, serves to attach a portion of the latter to the substratum, over 

 which the coenosarc creeps. 



Amoeboid ectoderm cells are responsible for the locomotion of 

 the young Corymorpha (Fig. 5) as of the adult, though they are 

 not confined to the proximal end of the stem. Often the latter 

 clings for half its length and may perform looping movements, 

 much less pronounced, however, than those of Hydra} The 

 direction of locomotion is also determined by the same factors 

 which regulate it in the adult. On a horizontal surface the direc- 

 tion is indeterminate, though the stem always moves out of its 

 investment. On an oblique surface it tends to move upward by 

 the nearest route. Young hydroids are often found adhering to 

 the stem of an adult, the relation of the axis of the attached por- 

 tion of each to the adult axis varying with the inclination of the 

 latter. If it is vertical, they are parallel with it, vertical also; and 

 the rest of the young stem will be nearly vertical, but not quite so 

 since the distal portion of the stem seems to shun any contact 

 (negative thigmotropism). If the orientation of the adult is 

 altered, the young hydroid will gradually take up a new position 

 in which the most distal point of attachment will be the greatest 

 possible distance above its proximal end. 



Not only, therefore, is the larva negatively geotropic with regard 

 to orientation, but this has a directive effect upon locomotion. 

 It is probably due to the effect of the stimulus of gravity on 

 the endoderm cells which line the single cavity and from which 

 the axial cells of the adult are derived. These cells do not con- 

 tain such enormous vacuoles as those in the axial endoderm, and 

 are ciliated. In these respects they resemble the parietal endo- 

 derm cells of the peripheral canals of the adult, which are their 

 descendants also. 



With reference to the amoeboid cells which produce locomotion 

 in Corymorpha, it may be recalled that the ectoderm of hydroids 

 is not uncommonly amoeboid. To cite but a single instance, not 



'C/. MarshaU. Zeitschr. f. w. Zool., XXXVII, p. 664. 



