62 THE BIOLOGY OF HYDRA : 1961 



The irregular profile of epithelial cell surfaces being presented 

 to the mesoglea could possibly reflect a mode of insertion into the 

 extracellular matrix, as suggested by Hess, Cohen and Robson (4). 

 Cell contacts across the mesoglea could be related to an attachment 

 function but could also be related to the transfer of nutrients from 

 gastrodermis to epidermis or to some mechanism of direct integra- 

 tion between the two muscle layers. 



In this paper I have attempted to present a brief account of 

 some of the various types of attachment that occur between epithe- 

 lial cells and between the epithelial cells and mesoglea in hydra. 

 The conclusion that all of these specializations represent kinds of 

 cellular attachment is based on comparison with other organisms 

 and on attempts to correlate structure with function. These attempts 

 take into consideration special physiological problems related to the 

 fresh water environment and the mode of feeding of these organisms. 

 A permeability barrier for the organism seems essential and attempts 

 to find a structural basis for this barrier have been unsuccessful in 

 the past. I have postulated that the septate form of desmosome could 

 be important in preventing the influx of excessive fluid to the inter- 

 cellular spaces (12). There is no direct evidence, however, that 

 septate desmosomes are any more effective in this respect than are 

 ordinary terminal bars found in ductile epithelium or gut of higher 

 forms. In fact, I am not sure that one can say positively that term- 

 inal bars function to preserve the intercellular milieu in any situa- 

 tion but evidence seems to favor such an interpretation. 



At end-to-end and lateral contacts of interdigitating muscle 

 fibers a strong adhesion is something which would appear essential 

 for the efficient transmission of force during contraction of the 

 muscle fibers. By the same token, the special kinds of attachments 

 of muscle to mesoglea might be expected in areas of particular 

 stress, such as presumably occurs at the bases of the tentacles. 

 All these forms of attachment must also be interpreted as having 

 importance for preserving relative cell positions during active 

 movements of the animal. The chemical or molecular organization 

 of the cell surfaces is certainly not yet known in suflicient detail to 

 permit conclusions about the actual mechanism of attachment either 

 between adjacent cells or between cells and mesoglea. I believe, 

 however, that additional information will be obtained through 

 further studies using techniques for dissociating cells and by using 



