14 W. J. CAIRD. 



If you remember when we were down at the shore, somebody 

 grabbed a hermit crab incautiously, and slightly pricked his ringers. 

 These prickles are the defensive persons of hydractinia. In this species 

 we have what may be termed a division of labour. The large polypes 

 with the proboscis and the long white tentacles are those which are 

 primarily charged with looking after the commissary department Their 

 sole function is to eat eat eat, and nothing but eat. Of course the 

 digested food passes as usual into the general body cavity and thus the 

 whole is nourished. 



Others take upon themselves the duty of playing the part of watch- 

 men. The head of the polype is rounded off very small, and the 

 tentacles are aborted, They cannot eat, but they have developed the 

 art of scenting danger from afar, and instantly the whole colony take 

 fright. Several times this summer I tried experiments on this hydroid, 

 which may be summed up thus. You may tickle the gormandiser pretty 

 freely and he does not mind you, but if you touch a sensitive person 

 ever so slightly the whole colony shuts up at once in the twinkling of 

 an eye. 



There are other persons yet each polype you must remember is 

 called a person which greatly resembles the sensitive person. These 

 feel also, but not to the same high degree. Near the head of these are 

 found numerous sporosacs in big clusters. Several medusoids emerge 

 from each sporosac. I have wondered why so many medusoids should 

 be developed, and only recently have I come to a finding satisfactory to 

 myself. Hydractinia always mark this always chooses the univalve 

 shell inhabited by the hermit crab. The other species we have con- 

 sidered seemed careless about their anchorage, any place would do so 

 long as it is safe. But not so hydractinia. It will only take the shell 

 inhabited by this crab or none. There are many suitable shells in the 

 sea, but if Mr Crab does not choose to live inside, it will not take up its 

 quarters there either. This seems to show a sort of reasoning power 

 at least a discriminating power of the free planula stage of the animal, 

 just the same as the ordinary white butterfly selects the cabbage on 

 which to deposit its eggs. You can easily find other examples ex- 

 pressed in different ways of this discriminating power, both in animal 

 and vegetable kingdoms, and doubtless too, you can give or manufacture 

 reasons why these should be so. 



So, too, I will hazard a guess why the medusoid stage of hydractinia 

 selects this inhabited shell. If it settled on the rocks, or on the weeds, 



