186 ANIMAL BEHAVIOUR 



amid a multitude of apparent alternatives, and (2) that it 

 is utilised in a fashion which is interpretable as peculiarly 

 adaptive. The first point may be illustrated by the case of 

 Technltella thompsoni, which covers itself with minute per- 

 forated Echinodenn platelets; the second point by the case 

 of Marsipella spiralis, which arranges its encasement of 

 sponge-spicules in a spiral, doubtless of considerable archi- 

 tectural value. 



5. Reflex Actions. 



Among simple multicellular animals we find, as among 

 the unicellulars, abundant illustrations of exploring, testing, 

 and hunting. Perhaps we may recognise more staying power, 

 persistence, and momentum, advantages naturally accruing 

 from the acquisition of a body. 



But the establishment of a nervous system opened the 

 way to the organisation of reflex actions, which are the out- 

 come of hereditarily prearranged linkages of nerve-cells and 

 muscle-cells. These play an important role in behaviour. 

 The sea-anemone's tentacles close upon their victim; the 

 nestling's mouth opens at the touch of the food in its mother's 

 beak ; the earthworm withdraws into its burrow when it feels 

 the tremor of a thrush's footstep ; we cough in spite of 

 ourselves when the crumb of bread is going the wrong way, 

 and so on. These reflex actions are uniform reactions to 

 a particular kind of external or internal stimulus ; they are 

 exhibited by all animals of the same kind in approximately 

 the same way, though some individuals are quicker than 

 others; they are independent of individual experience and 

 do not require control on the part of the central nervous 

 system; they depend on inborn structural linkages of par- 

 ticular sensory and particular motor nerve-units or neurons. 



