TRANSMISSION IN THE NERVE-NET 135 



cleavage or grain. Consequently receptive cells, instead 

 of being closely associated with, the muscles that they 

 control, may come to lie at some distance from them and 

 thus arises the necessity for extended transmission. This 

 is clearly seen in the contrast between the tentacles and 

 the general retractor mechanism of actinians. In the ten- 

 tacles a system of ectodermic sense cells immediately over- 

 lies the ectodermic longitudinal muscle and hence conduc- 

 tion from the sense cells to the muscle is simple and direct. 

 In the retractor mechanism, on the other hand, an ecto- 

 dermic system of sense cells calls into action a distant 

 entodermic system of retractor muscles in the mesenter- 

 ies, and this involves not only a nerve-net but a net of 

 such dimensions as to accomplish the extended transmis- 

 sion necessary for the performance of general retraction. 



Such modifications of the nerve-net lead to conditions 

 in which are realized the beginnings of unquestionable re- 

 flex activity. In this form of response a definite motor 

 or other efferent activity appears after the application 

 of a specific stimulation. In other words, on the applica- 

 tion of a definite stimulus not a diffuse response but a 

 highly particularized one appears. This is well seen in 

 certain reactions that depend upon oral and tentacular 

 stimulation in the sea-anemone Metridium. 



If a small amount of seawater is discharged into the 

 mouth of an expanded, resting Metridium, no response is 

 usually noticeable. If the seawater contains hydrochloric 

 acid -j^- , the actinian immediately opens the oesophagus 

 widely and exhibits on its column a few well-marked ver- 

 tical grooves. These disappear gradually as the oesoph- 

 agus closes. If the position of these grooves is carefully 

 noted, it will be found that one is always present for each 

 siphonoglyph and that the others are distributed in ac- 



