156 THE ELEMENTARY NERVOUS SYSTEM 



changes, as maintained by Nagel (1894), cannot be stated 

 at present. 



Many observers in the past have noted that retracted 

 sea-anemones can be induced to expand by placing pieces 

 of meat or other food so near them in the water that dis- 

 solved materials from this food are wafted to the animals. 

 Pollock (1883) and Romanes, in consequence of such ob- 

 servations, were led to assume the presence of the olfac- 

 tory sense in these animals. More recently this response 

 to food has been observed in Metridium by Allabach 

 (1905) and in Actinia by Pieron (1906 b, 1906 c). Alla- 

 bach 's statements can be confirmed easily on Metridium. 

 If into two large glass dishes of fresh seawater many 

 specimens of contracted Metridium are placed and into 

 one of these dishes is poured a small amount of juice 

 from a crushed mussel, the sea-anemones in that dish 

 almost without exception will expand their oral discs in 

 a very few minutes, whereas those in the other dish will 

 remain almost to an individual retracted. It was quite 

 clear from observations of this kind that the dissolved 

 products from the food of the sea-anemone would induce 

 the expansion of its oral disc, though this agent had very 

 little effect on the shortened condition of the column in 

 these animals. 



The part played by oxygen in the expansion and re- 

 traction of sea-anemones has been a matter of recent dis- 

 pute. According to Pieron (1906 fc), Actinia equina opens 

 in seawater with a large oxygen content and closes when 

 there is a deficiency of this gas. Pieron (1908 a, 1908 fc, 

 1909), as a result of further investigations, was led to 

 believe that not only did oxygen have this effect but that 

 it was one of the most important factors in determining 

 expansion and retraction. Bohn (1908 a, 1908 c, 1910 a), 



