Biological Studies on Coryniorpha. 417 



grains, as a plant root pushes its way through the earth. In no 

 case, however, does it appear to respond to the stimulus of gravity, 

 or any other stimulus, except that of contact. Resistance seems 

 to incite movements which overcome it. This is probably the 

 reason why the filaments leave the easy path between stem and 

 perisarc to push out against the resisting wall of the latter. 



d. Circulation; Cilia. 



A fourth type of motion is found in the currents set up in the 

 cavities of the digestive tract by means of cilia. The cilia are 

 borne on the lining cells of the proboscis of the hydroid and the 

 epithelium bounding the peripheral canals on their outer side. 

 They are present throughout the peduncles bearing the medusae, 

 and the manubria of the latter. 



There are variations in the currents, particularly in the peri- 

 pheral canals. At times there may be no current at all. At 

 others the current may be setting very rapidly in the same direc- 

 tion in all the canals visible. Abrupt reversals occur under these 

 conditions, which can hardly be explained by ciliary action, but 

 are rather the result of expansions or contractions of the proboscis 

 and stem, which produce changes of pressure in the canals. 



IV. THE YOUNG HYDROID. 



The eggs are laid by medusae which are never set free from 

 the hydroid. They are small but heavy with yolk and fall directly 

 to the bottom in quiet water, adhering by their delicate coats to 

 the first object they touch. 



As soon as the egg is attached, its free life is practically over. 

 The embryo is never ciliated^ and has no free-swimming phase in 

 its existence. It is capable of very slow creeping movem'ents, 

 however, by means of which it often comes in contact with other 

 embryos and forms with them temporary associations of as many 

 as six, ten, twelve individuals. Often it will travel many times its 

 own length, leaving behind a narrow collapsed tube of perisarc 



iC/. Hypolytus peregrinus, Murbach (Q. J. M. S., XLII, 1899, p. 341), which it 

 resembles in this and other respects. 



