116 Papers from the Department of Marine Biology. 



LOCOMOTION. 



Like most of the comatulids hitherto studied, Tropiometra is exceed- 

 ingly sluggish. So far as my observations go, it never swims. Dropped 

 or suspended in deep water, it makes only the most feeble arm-move- 

 ments and, unless artificially prevented, it sinks rapidly to the bottom. 

 When, as a result of some adequate stimulus, Tropiometra does move, 

 the movement is similar to that of Comanthus. That is, it is accom- 

 pUshed solely by means of the arms, and no particular arms take pre- 

 cedence in the matter. In one respect the attitude differs from that 

 of Comanthus, for the l3ody is much more distinctly raised. This is 

 obviously to keep the cirri wholly above the substratum and thus 

 prevent their accidentally catching on it. The movement is always 

 slow and labored and does not continue long. When overturned so 

 that the cirri are up, a normal healthy Tropiometra invariably rights 

 itself at once, sometimes only very slowly and deliberately, but usually 

 without delay. The process is, as in other comatulids, by several 

 arms raising up their side of the body until one or more of them or 

 some of the neighboring arms can reach over backwards and secure 

 a hold which will enable them to pull the body over. The cirri play 

 no part whatever in this process, any more than in actual locomotion ; 

 they are very inert, move slowly and slightly, and are used only as 

 organs of attachment, a purpose which they serve admirably. The 

 pinnules, even near the tip of the arm, have only weakly hooked tips 

 and are but slightly viscid, and it is consequently difficult for Tropio- 

 metra to move or even to right itself except on a rough and rather firm 

 surface. No individuals were seen moving about under normal con- 

 ditions, but when placed on a bare, sandy bottom they crept steadily 

 to its margin of coralline algse. Many specimens were planted on the 

 reef-flat near the laboratory and most of these ultimately disappeared, 

 but whether they crept away or were carried off by tidal movements 

 it is impossible to say. 



REACTIONS TO LIGHT. 



The remarkable habitat of Tropiometra in Buccoo Bay naturally 

 suggested that in its reactions to light it would be positively photo- 

 tactic. A tank 6 feet long, a foot wide, and 8 inches deep, having the 

 bottom covered with broken coral and stones, was used for the experi- 

 ments. It was placed near high-water mark on the beach, in the sun, 

 and two boards, 5 or 6 inches wide, were laid across it, giving shaded 

 areas; one end was made quite dark by covering with a broad piece 

 of board and burying it in the sand. Numerous individuals of Tropio- 

 metra were tested in this tank on many different occasions and under 

 diverse conditions of temperature. The results were surprisingly 

 unsatisfactory. No individual showed any decided reaction to light 

 and none sought the darkness. In most cases, when the specimen 



