356 DR. N. ANNANDALE ON 



their greatest length during the heat of the day. Frequently the tentacles, when in this 

 condition, are so* arranged as to resemble a net bellying out in one direction. They hang 

 down close and parellel to one another and are all curved in the middle, just as though a 

 current was directed against them in the water. When they are in this position, prey 

 which chance to swim amongst them, has little chance of escape. Before assuming 

 the attitude they hang down in the water, or are occasionally directed upwards, 

 being held straight except for a slight outward flexure at the base. It is not un- 

 common, however, for one of the tentacles to remain contracted while the others are 

 fully elongated. In some cases it appears that a specific tentacle has a greater tendency 

 to contract than the others ; but whether this tentacle is the one produced latest I have 

 been unable to ascertain, and the matter is one which requires further observation. 



Movements. 



The question of the movements of Hydra is one which has been very thoroughly 

 investigated as regards European and North American species, with practical unani- 

 mity on the part of the investigators as to the main features of progression. There are 

 two points, however, in which Hydra orientalis differs in this respect from northern 

 forms : (i) it moves away from light, w T hile they are attracted towards light ; and (2) it 

 does not progress by ' ' looping ' ' as they do, but by alternately expanding and con- 

 tracting its body without raising it from the surface on which it crawls. 



That the European Hydrae move towards light was observed by Trembly (whose 

 classical memoir I have, unfortunately, been unable to consult) and other early ob- 

 servers, while more recently E. B. Wilson, 1 and G. Wagner 2 have published details show- 

 ing the actual course taken by an individual in a given time. If a number of specimens 

 of Hydra orientalis be placed in a glass jar half of which is illuminated while the half is 

 in shadow, they will be found, after a few hours, to have deserted the bright side and 

 taken up a position on the shady one, not all together but scattered rather widely apart. 

 The majority will also make their way towards the bottom. They appear to move more 

 nearly in a straight line than northern species do, and, as soon as they have escaped 

 from the bright light, they settle down and often remain in the same position for 

 the rest of their lives. It is only an unfavourable change in conditions such as a 

 rise of temperature, or fouling of the water, that causes them to move again. In natural 

 circumstances buds rarely move far from their parents unless such changes occur. 

 Young individuals, however, appear to change their situation more readily than older 

 ones, and hence possibly arises their greater immunity to unfavourable conditions, 

 the older ones often perishing because they do not move away from an unsuitable 

 situation. I have already pointed out that migrations take place in the " tank " in 

 accordance with changes in the weather. Probably heat is the real cause of the polyp's 

 apparent dislike of light. 



The modes in which such migrations are performed are at least three : (1) by 

 the polyp fixing itself to a mollusc, (2) by its crawling, and (3) by its floating. The last 

 mode is adopted mainly by young individuals, which expand both their tentacles and 



1 Amer. Naturalist, XXV. p. 413, 1891. 2 Quart. Journ. Micr. Sci. XLVIII, 1905, p. 587. 



