PHYSIOLOGY OF SYNAPTULA HYDRIFORMIS 339 



the side of a vessel or along the bottom. When a Synaptula, in 

 creeping up a vertical wall, reached the surface of the water, it 

 generally began moving horizontally, say to the left, attaching 

 its tentacles more or less in regular sequence, and rolling over, 

 till the lighter side of its body was away from the wall. Then, 

 instead of making a complete revolution and continuing to the 

 left, it would 'roll' back to its starting-point and over to the right, 

 until again the lighter side appeared. It would thus roll back 

 and forth some five or six times before it finally loosened hold 

 and dropped to the bottom. Similarly, while crawling along the 

 bottom of a dish, a Synaptula usually rolls back and forth, very 

 seldom moving in a spiral. 



Upon dissection this lighter colored side, the one which is 

 kept next the substrate, was fqund to be that which, for morpho- 

 logical reasons, has been called ventral. Aside from the differ- 

 ence in color and in abundance of miliary granules, differences 

 which are evident in both red and green varieties, there are no 

 external differences between dorsal and ventral surfaces. 



The Holothuria can be arranged in a series according to in- 

 creasingly pronounced bilateral symmetry and physiological 

 polarization. One of the lowest members of such a series is 

 Thyone briareus, which has tube-feet along all five of its anti- 

 meres, and moves with any side forward (Pearse, '08; Mast, '11, 

 p. 211). Holothuria surinamensis and H. rathbuni, according 

 to Crozier ('14 a, J 14b), are intermediate forms, while Holo- 

 thuria captiva occupies the upper end of the series, since it has 

 tube-feet on the ventral side ('trivium') only and always moves 

 with the oral end forwards (Crozier, '14 b). Synaptula hydrifor- 

 mis would rank with Holothuria captiva, since, although its 

 bilateral symmetry is not so pronounced, its physiological polarity 

 is most marked. 



2. Feeding and digestive movements 



In spite of the fact that this Synaptula moves with one par- 

 ticular side next the substrate, its rolling motion brings all the 

 tentacles into play, even those on the dorsal surface. In its 

 natural habitat in sea-weed, all the tentacles are used in locomo- 



THE JOURNAL OF EXPERIMENT AL ZOOLOGY, VOL. 24, NO. 2 



