FOOD AND THE NERVE-NET 139 



usual outward stroke and thus transport such materials 

 to the digestive cavity. Besides these five sets of parts 

 some actinians include in the means by which they 

 appropriate their food a sixth system, namely, the mus- 

 culature of the oral disc. In Stoichactis, for instance, as 

 described by Jennings (1905), and in Cribrina, as re- 

 ported on by Gee (1913), the mouth during feeding is 

 moved by the oral musculature toward the food-bearing 

 tentacles, a shifting which has also been observed in cer- 

 tain corals (Carpenter, 1910). This operation, though it 

 can be seen to occur in Metridium, is relatively so insig- 

 nificant in this form that it may be passed over without 

 comment; the important elements in the feeding of this 

 actinian are the five already mentioned. 



Much confusion and uncertainty exists in the various 

 accounts of the methods by which actinians obtain their 

 food and more or less of this is due to the failure on the 

 part of writers to designate the particular form of ac- 

 tivity that they are for the moment discussing. Thus 

 both ciliary and muscular activity are involved in the 

 appropriation of food and have often been indiscrim- 

 inately dealt with in accounts of this operation. Their 

 significance for the animal as a whole is, however, very 

 different and it is, therefore, highly desirable that they 

 should be; kept clearly in mind as separate processes in 

 any discussion in which they are involved. 



Of the five principal events that go to make up the act 

 of food appropriation, three exhibit so little variation that 

 they may be regarded as essentially uniform. These are 

 the secretion of mucus, the beat of the tentacular cilia, 

 and the opening of the oesophagus. In none of these are 

 there during feeding any important readjustments which 



