NERVOUS TRANSMISSION 97 



layer of the ectoderm is best developed in the region of 

 the lips, but, as experimental studies have shown, this is 

 precisely the part of the animal that is poorest in nervous 

 conduction. It, therefore, seems probable that, though 

 the so-called nervous sublayer of the Hertwigs must be 

 penetrated by a host of neurofibrils from the overlying 

 sensory epithelium, this sublayer must have some other 

 function than that of nervous conduction, which must be 

 carried out by another part of the animal. The part most 

 probably concerned with nervous conduction, judging 

 from the discoveries of the last few years, is the support- 

 ing lamella. This layer has long been known to contain 

 a network of cells and from the work of Havet (1901) and 

 of Parker and Titus (1916) the richness of its nervous con- 

 nections can no longer be doubted. It would, therefore, 

 not be surprising if future investigation should show that 

 the chief mass of the actinian nerve-net should be found 

 to be located in the supporting lamella and not in the so- 

 called nervous sublayers of the ectoderm and the 

 entoderm. 



The rate at which the nerve-net of actinians transmits 

 impulses has been determined for Metridium (Parker, 

 1918 b). When large specimens of this sea-anemone are 

 prepared as shown in Fig. 26, and the stimulus is applied 

 at the distal end of the tongue of tissue cut from the 

 pedal edge, the longitudinal mesenteric muscles contract 

 vigorously. If now the stimulus is applied to the root 

 of the tongue, the muscular contraction occurs in less time 

 than in the former instance. The difference between these 

 two periods of time thus determined is the interval neces- 

 sary for transmission between the two points stimulated. 

 As the distance between these two points can be measured 

 with accuracy, the rate of transmission can be easily de- 

 7 



