NEUROMUSCULAR MECHANISMS 155 



nected by synapses with two-way conduction and with no noticeable 

 synaptic delay, making it possible to consider these systems as simple 

 motor-nerve fibers. As a consequence the facilitations observed have then 

 to be located at the myoneural junctions. 



From anatomical evidence of Semal-van Gansen (1952a,b) it seems 

 somewhat doubtful whether in Hydra the same differentiation is respon- 

 sible for the difference in reactions shown by the longitudinal and circular 

 muscles, which are here ecto- and endodermal, respectively. The muscle 

 fibril bundles are found to be located in the basal parts of the cells. The 

 ectodermal bundles are in juxtaposition with each other, and more than 

 one bundle may belong to one ectoderm cell. In the transverse fibers of the 

 endodermal cells there is only one muscle fibril bundle per cell and these 

 are usually well separated from each other. The nervous system consists 

 in the ectoderm of multipolar cells, which may fuse, while in the endoderm 

 the nerve cells are bipolar without visible connections. There may thus be a 

 relation between these anatomical findings and the rather quick contrac- 

 tion of the longitudinal, ectodermal layer and the much slower stepwise 

 contractions of the circular, endodermal fibers. 



In sea anemones, Batham and Pantin (1954) and Passano and Pantin 

 (1955) have found further confirmation of the differences in reactions of 

 the different muscle layers. It has especially become clearer that the slow 

 contractions and with them the slow movements of the animal as a whole 

 may be based on spontaneous activity in the muscles. 



Investigations in Tunicates 



Tunicates have been investigated by Hoyle ( 1952) with methods similar 

 to those used by Pantin on sea anemones, and some remarkable resem- 

 blances were shown to exist. But, as Hoyle has pointed out, until one can 

 be more certain of the alleged underlying factors, it must necessarily re- 

 main doubtful how far the phenomena observed find their origin in the 

 neuromuscular transmission. It is nevertheless remarkable that animals 

 which are phylogenetically presumably so far apart seem to have many 

 similarities in mechanisms. The sessile mode of living may be part of the 

 explanation, but other sessile animals clearly have different mechanisms, 

 or at least still show in part of their behavior properties which the free- 

 living related forms possess (e.g., barnacles). 



Conclusion 



Some very gratifying advances have been made in the area covered by 

 this review. In one respect, though, the recent results may be somewhat 

 discouraging, since they have shown that slight changes in the properties 

 of neuromuscular transmission result in great differences in readily ob- 



