TEANSMISSION IN THE NEKVE-NET 137 



agus will still open, accompanied by the formation of ver- 

 tical grooves on the column. This response can not be 

 elicited by the application of weak acid to the tentacles. 

 Under such circumstances a withdrawal of the. oral disc 

 takes place. Thus it appears that not every form of 

 effective chemical stimulus that can be applied to the ten- 

 tacles is followed by an opening of the oesophagus. To 

 appropriate stimuli, however, the tentacles and lips may 

 act as receptors for the opening of this tube. No other 

 parts of the bodj^ of Metridium have been found from 

 which this cesophageal response can be called forth. The 

 response, therefore, partakes of the nature of a true re- 

 flex in that it implies definite transmission tracts from 

 the tentacles and from the lips to the transverse muscles 

 of the mesenteries, and these tracts are called into activ- 

 ity only by specific forms of stimulation. Thus while 

 dilute acid applied to the tentacles will excite the general 

 nerve-net of Metridium, fish meat applied to these organs 

 calls forth the oesophageal response only. Jordan (1908) 

 has claimed that animals possessing only nerve-nets in 

 their nervous organization exhibit reflex deficiency and 

 this in general is undoubtedly true, but, as the present 

 example shows, some of these animals may have involved 

 in their nerve-nets specialized tracts that enable them to 

 carry out simple but obvious reflexes. Such conditions 

 indicate the steps by which a nerve-net may be converted 

 into that type of central nervous organ that is char- 

 acteristic of the higher animals (Parker, 1916 a, 1917 d). 



