96 THE ELEMENTARY NERVOUS SYSTEM 



longitudinal mesenteric muscles, by whose contraction 

 the oral disc is depressed, may be called into action from 

 almost any part of the outer surface of the sea-anemone. 

 Paths of conduction must, therefore, exist between prac- 

 tically every point of the exterior of the animal and the 



longitudinal muscles of 

 the mesenteries, and for 

 these paths of conduction 

 the oral disc and the 

 oesophagus are in no way 

 essential. In fact, definite 

 and circumscribed paths 

 appear not to exist, but, as 

 the directions of the vari- 

 ous incisions and cuts 



FIG. 29. Metridium, seen from the oral shOW, it is nCCCSSary to aS~ 



pole, cut through vertically except for a 



email connecting bridge on the pedal edge SUniC, first, an immediate 

 of the column. Stimulus applied at z. 



connection between the 



superficial ectoderm and the deep-lying musculature, and, 

 next, a veritable nervous network by which all manner of 

 irregular and complicated incisions can be circumvented 

 so long as a bridge of the natural tissue remains as a 

 means of connection between the part stimulated and the 

 part responding. In other words, some of the most char- 

 acteristic reactions of sea-anemones are to be ascribed 

 to the fact that these animals possess a well-differentiated 

 nerve-net (Parker, 1917 a}. 



The location of this nerve-net is by no means clearly 

 determined. At first thought it might be expected to occur 

 in the nervous sublayer described in the ectoderm and 

 in the entoderm by the Hertwigs. But doubt has already 

 been expressed as to the nervous character of this layer 

 (Parker, 1912). In Metridium the so-called nervous sub- 



