ORGANISM AND MECHANISM 123 



compelling constitutional homesickness. Professor Yung of 

 Geneva took twenty bees from a hive near the lake, put them 

 into a box, and carried them six kilometres into the coun- 

 try, where he set them free. Seventeen returned to the hive, 

 some of them in an hour. 



The fresh-water mussel carries her young ones in her outer 

 gill plate, and keeps them there, even long after they are 

 ready to emerge, until a stickleback or a minnow comes into 

 the immediate vicinity. When the fish comes near, the 

 mother-mussel, whom it is no libel to call * acephalous ', 

 liberates a crowd of the pinhead-like larvae, which swim out 

 into the water, snapping their tiny toothed valves, and 

 secreting viscid attaching threads. They have the begin- 

 nings of a nervous system ; they are sensitised to some 

 stimulus from the fish; they fasten on to it and begin an- 

 other chapter of their life. Even in the laboratory, when 

 they have been removed from the mother, they become ex- 

 traordinarily excited if a morsel of minnow be dropped into 

 the dish in which they are. They respond definitely to the 

 only stimulus which will enable them to continue their life. 

 In some North American fresh-water mussels only one par- 

 ticular kind of fish will serve the purpose of temporary host. 



In the remarkable life-history of the liver-fluke of the 

 sheep, microscopic ciliated larvae emerge from egg-cases 

 which have fallen into w^ater. These larvse have no organs 

 in the strict sense, no hint of a nervous system, and only a 

 few cells altogether. They have energy enough to go on 

 swimming for about a day in the water-pool. They may 

 come in contact with many things, sticks and straws, roots 

 of aquatic plants and various aquatic animals, but there is 

 (in Britain) only one touch to which they respond — that 

 of the small fresh-water snail, Limncea truncatula, the only 



