BEHAVIOUR OF SHORE ANIMALS 221 



Fleure and Walton (1907), from a study of Actinia^ 

 Tealia and Anthea, conclude that the base of the animals and 

 the tentacles are the parts most sensitive to mechanical 

 stimuli. In Anthea and Actinia the mouth is the region 

 specially sensitive to chemical stimuli, and the tentacles are 

 insensitive except when the chemical stimulus is excessive. 

 The behaviour of these forms would seem to be modifiable 

 by experience. Thus, pieces of filter paper placed on some 

 of the tentacles of an Actinia were carried to the mouth, 

 but, after a few trials lasting over two to five days, similar 

 offers were rejected by the tentacles. 



According to Jennings (1905), changes in the internal 

 metabolism, former stimuli that have affected the animal, 

 previous reactions performed and other factors, combine to 

 determine the action of a sea-anemone. There is also a 

 marked tendency in some cases to repeat an action in the 

 way it has been performed oh previous occasions. These 

 various factors give, according to this writer, a high degree 

 of complexity and adaptiveness to the behaviour of even 

 these lowly animals. 



A structure of particular interest in sea-anemones is the 

 stinging cell or cnidoblast. These have been described 

 already (p. 105), but we may refer here to the fact that each 

 stinging cell possesses a special sensory process or cnidocil, 

 stimulation of which provokes the liberation of the stinging 

 thread. The fact that the bases of the stinging cells branch as 

 do those of the ordinary sensory cells led observers to beheve 

 that the stinging cells were subject to nervous control in 

 the same way as the muscles of these animals. Apparently, 

 however, there is no experimental evidence in support of 

 this view, and it must therefore be concluded that the 

 nematocysts are only discharged under direct stimulation 

 from the exterior (Parker, op. cit.). 



Hydroids. — From an experimental study of Corymorpha, 

 Parker {op. cit.) concludes that the nervous organisation of 

 hydroids is very much like that of anemones, but represents 

 a somewhat simplified condition ; it is characterised by the 

 same lack of any general nervous centre and the same 



