6 THE BIOLOGY OF HYDRA : 1961 



tioned above, all the eells are probably embedded in mesoglea. 

 These points of contact between the muscle layers are fairly fre- 

 quent and occur in all areas investigated. 



THE RELATIONS OF CELLS TO EACH OTHER 



The special relationships of muscle cells and the fact that all 

 cells appear embedded in mesoglea have already been discussed. 

 However, there are other peculiar relations of cells that should be 

 mentioned. By no means are the limiting membranes of the cells 

 smooth. At times, a button or snap fastener arrangement can be 

 seen where one cell evaginates a piece of cytoplasm to rest in an 

 indentation of an adjacent cell ( Fig. 2 ) . This causes the frequent 

 appearance of circular areas of cytoplasm located between cells. 

 In addition, terminal bars are seen between some cells lining the 

 lumen (Fig. 14) and between other cells near the mesoglea 

 (Fig. 19). 



NERVE CELLS AND FIBERS 



No cell was foimd which could be called a nerve cell. As 

 explained above, the small circles located between cells, which 

 may sometimes form clusters and appear like bundles of nerve 

 fibers (Figs. 15, 18, 21, 26), probably result from the peculiar 

 formations of the cell borders. There is the possibility that 

 nerve tissue of Hydra may appear different in the electron micro- 

 scope from that of other organisms, and we are thus unable to 

 identify nerve cells or fibers in our electron micrographs. However, 

 if the absence of nerve tissue in Hydra can be accepted, one may 

 perhaps go further and wonder if, indeed. Hydra needs any nerves. 

 The epithelio-muscular cells containing the muscle filaments are 

 on the surface of the animal and there can act as receptor cells 

 to lead the impulse to its muscle filament organelles. The impulse of 

 one ectodermal muscle fiber could easily be transmitted to muscle 

 filaments in adjacent epithelio-muscular cells. Endodermal muscle 

 could conduct an impulse from one cell to another in a similar 

 manner. Lastly, the interaction of ectodermal and endodermal 

 muscle could well be achieved through the points where mesoglea 



