212 



HYDROIDiE. 



Part IV 



Fig. 29. 



Fig. 30. 



the quick contractions, which press the water out of the main cavity with such 

 force as to push rapidly the whole body forward in an opposite direction. After 

 each contraction, and during the onward movement arising from it, the tentacles 

 point directly backward. During each contraction they are considerably shortened, 

 and elongate gradually in the progress of locomotion. 



This animal seems very well to understand how to direct its course by its con- 

 tractions, as it darts dowuAvard if it be near the surface of the water when starting, 

 or moves sideways if it be near the walls of the jar, or rises upward if it be at its 

 bottom. It may suddenly change its direction, if it meets with an obstacle, turn 

 once or twice upon itself, in a revolving curve, and then dart again, suddenly, straight 

 forward, in any given direction. Of course, the changes of form which it assumes, 

 in these different movements, are almost endless.^ What 

 increases the variety of its aspects beyond the change 

 of form of the main body, the shortening and elongating 

 of the tentacles, and the shutting and opening of the 

 main cavity, is the disposition of the proboscis, which is 

 either entirely contracted within the main cavity, near 

 its upper centre, or hangs down to the margin of the 

 opening, or stretches out between the tentacles to two 

 or three times the length of the body (wood-cut 29, d), 

 in either a straight line, or variously bent in graceful 

 undulations, or curved upon itself (wood-cut 30). Though 

 the usual form of these animals is rounded, it may be 

 seen at times to contract in such a manner as to 

 assume a flattened shape in its lower part by the compression of its 

 sides; and this is especially the case when the animal turns round 

 upon itself, and changes its direction in its movements, 

 or the bell elongates to such an extent as to become 

 cylindrical and twice as high as broad (wood-cut 31). 

 Again, when it pauses, and remains in a state of rest for a longer time, 

 the lower margin is frequently seen to assume a square or quadrangular 

 form; especially when it is perfectly immovable, and the tentacles are 

 stretched out at right angles from the lower margin for a considerable 

 length (PI. XVllI. Fig. 17). On watching minutely its outline, it will be observed 

 that the sides are not always circular, but from the contraction of the layers or 

 bundles of motory cells, it assumes a quadrilobate appearance (PI. XVIII. Fig. 18). 



id' 



Fig. 31. 



* A very full description of this species may 

 be found in my paper on Aealephs, in the 4th Vol. 



of the Memoirs of the Amer. Acad, of Arts and 

 Sciences, 18C0, with numerous tigures. 



