CHAPTER XIII 

 RESISTANCE TO HYPOTONIC SEA-WATER 



We have just seen that when exposed to a variety of toxic agents, 

 with the mass of animals in optimal relation with the volume of the 

 medium and other conditions being favorable, groups of animals- 

 will survive longer than will equal numbers of single individuals 

 when each is isolated into the same volume of the same medium to 

 which the group is exposed. Our experience with colloidal silver 

 and with several other reagents indicates that such protection is 

 largely, and at times probably completely, furnished by the fixa- 

 tion of the toxic substance either directly by the mass, or by shme 

 and other products given off, so that it is either removed or dis- 

 tributed among so many individuals that each receives a sublethal 

 dose. If the animals are closely aggregated, only those on the out- 

 side receive the full impact of the toxic agent, so that those within 

 are protected by another type of mechanical action of the mass. 

 Such an attack on the problem of the protection of the individual 

 by the mass, while demonstrating the fact of the protection, does 

 not furnish critical evidence concerning the production of an auto- 

 protective secretion other than slime, which in these cases presum- 

 ably owes its protective action to its adsorptive power. 



A better opportunity to obtain critical evidence is furnished when 

 the toxic properties of the solution are due to the absence of easily 

 measured materials rather than to the presence of some toxic sub- 

 stance added to the water. Such conditions are fulfilled when marine 

 animals are placed in hypotonic sea-water. In concluding a note on 

 the effect of exposing the marine turbellarian Convoluta roscofensis 

 to hypotonic sea-water, Bohn and Drzewina state (1920): 



"Toutes conditions egales d'ailleurs, les individus isoles sont in- 

 finiment plus sensibles que les individus groupes, comme si le fait 

 d'etre groupes constituait pour eux une protection. Le contraste 

 est souvent saisissant. Soient deux verres de montre, dont I'un 



