NEUROMUSCULAR MECHANISM IN METRIDIUM 453 



IV. DISCUSSION 



The neuromuscular mechanism in Metridium when studied 

 in detail presents much greater variety than previous investi- 

 gations have described. There is no uniform plan of organiza- 

 tion in these important structures that applies generally to 

 the whole animal but a variety of conditions obtains. At least 

 four types of organization occur. First there are muscles whose 

 action appears to be quite independent of nervous control such 

 as the longitudinal muscles of the acontia. These muscles are 

 extremely slow in their responses; their contractions are known 

 to follow a mechanical stimulation only after an interval of a 

 minute or so, and their activity continues even after the tissue 

 in which they are imbedded has been permeated with an anes- 

 thetic. These muscles may be said to represent the group of 

 independent effectors, such as are seen in the muscles of sponges, 

 in the embryonic vertebrate heart, and so forth. 



A second type of neuromuscular organization is seen in the 

 circular muscles of the column wall and possibly also of the ten- 

 tacles. These, like the acontial muscles, are normally open to 

 direct stimulation but are probably also somewhat under nervous 

 control. 



A third type of neuromuscular organization is seen in the 

 tentacular ectoderm of Metridium. Here peripheral sense 

 cells are closely associated with muscle fibers. This system 

 acts with relatively great rapidity, a contraction following a 

 stimulation in less than a second. It also exhibits nervous 

 conduction. Complete anesthetization abolishes its activity 

 and leaves the muscle incapable of responding to ordinary stim- 

 uli. In its essentials it may be said to consist of receptors com- 

 bined more or less immediately with effectors. 



The fourth type of neuromuscular organization in Metridium 

 is such as is seen in the system including the sense cells of the 

 column wall, the longitudinal muscles of the mesenteries and the 

 connecting tracts. This type brings into physiological relation 

 distantly located parts. Like the third type its activities are 

 quickly carried out and are also easily obliterated by anesthetics. 



