NERVOUS TRANSMISSION IN THE ACTINIANS 93 



difTerentiated transmission tracts of the higher animals. Whether 

 this differentiation takes the form of semi-independent nerve 

 nets, as suggested by von Uexkiill ('09), or is of the nature of 

 somewhat isolated and, in consequence, nerve-like tracts which 

 connect one semicenter with another, cannot be stated. 



SUMMARY 



1 . Nervous transmission may be accomplished from almost any 

 part of the ectoderm of Metridium to its longitudinal mesenteric 

 muscles. 



2. Experiments in which the receptive part of a Metridium is 

 connected with the effector portion by only a small bridge of 

 tissue demonstrate that these nervous connections occur in such 

 a variety of positions as to call for the assumption of a nerve 

 net. 



3. These connections in many places pass directly from the 

 ectoderm, through the mesogloea, to the entoderm. In con- 

 necting the ectodermic with the entodermic systems the lips, and 

 probably the esophagus, are not as irnportant organs as many 

 other parts of the body. 



4. Notwithstanding the generally diffuse condition of the 

 transmission system in Metridium, there is evidence also for a 

 certain degree of specialization in this system. Stimulation of 

 the tentacles by mussel juice calls forth a gaping of the esophagus 

 (contraction of the transverse mesenteric muscles) and by weak 

 hydrochloric acid a retraction of the oral disc (contraction of the 

 longitudinal mesenteric muscles). These two forms of response 

 afford good ground not only for the assumption of independent 

 receptors but for the belief in relatively independent transmission 

 tracts, a first step in the kind of differentiation so characteristic 

 of the nervous organization in the higher animals. 



