BEHAVIOUR OF SHORE ANIMALS 243 



These activities of Foraminifera and tube-worms are 

 " instinctive " in the sense that they are displayed indepen- 

 dently of experience and in equal measure by all the members 

 of a particular group, but the use of this term leads us no 

 nearer an understanding of the mechanism underlying 

 them. 



The nest of the stickleback as a piece of workmanship 

 is of a considerably lower order than that represented by the 

 shell, let us say, of the Foraminiferan Marsipella spiralis 

 (in which the spicules are twisted into a left-handed spiral, 

 so increasing the strength of the shell) or the tube of Amphic- 

 tene (see p. 103), but the behaviour of the stickleback 

 may well be on a higher level notwithstanding. For the 

 form of the Polychast tube follows closely that of the body, 

 and direct stimuli of contact, pressure, and so on, are possibly 

 of importance in determining the placing together of the 

 grains. The nest-building of the stickleback, on the other 

 hand, like that of birds, involves a much more diverse series 

 of operations : active search for the proper weeds, removing 

 them when found, weaving them together and securing them 

 with threads of mucus and so on. 



Occupation of Crasteropod Shells by Hermit Crabs. — 

 Not only does the hermit crab require to gain admittance 

 into a univalve shell when it first becomes adult, but as it 

 increases in size, larger shelters have to be looked for. 

 The question therefore arises as to what extent hermit 

 crabs may be said to '* recognise " the shells likely to prove 

 suitable for them, and by what means this recognition is 

 accomplished ? 



According to Bohn (19 10), a hermit crab in the task of 

 finding a suitable shell is aided, not by its powers of vision, 

 but by an extraordinarily well-developed tactile sense. No 

 sooner does one of the crab's limbs chance to come in contact 

 with a shell than the crab at once applies its body to it with 

 the head downwards and, getting a support for the chelas 

 on the lower part of the shell, causes the latter to rotate so 

 as to bring the orifice into a convenient position. The 

 animal then sounds the orifice with its chelae and eventually 



