RESISTANCE TO HYPOTONIC SEA- WATER 223 



contient, dans I'eau deluee a 75 pour 100, quelques individus, et 

 I'autre plusieurs centaines de Convoluta; les premiers sont cytolyses 

 en quelques heures, les derniers apres plusiers jours." 



Elsewhere they state clearly their belief that the observed pro- 

 tection is due to the secretion of an autoprotective substance by the 

 mass in greater quantity in proportion to the available volume of 

 water than is possible for the isolated animals. 



Lapicque (1921), in discussion, made the obvious objection that 

 the introduction of differing numbers of marine animals into the 

 same quantity of hypotonic sea-water would have a differential ef- 

 fect upon the salt content and that the difference in survival might 



Fig. 16. — Procerodes wheatlandi, dorsal view. The posterior end forms a muscular 

 sucker by means of which the animal attaches to the under side of rocks near the low 

 tide line. These animals are usually found in considerable numbers on a given stone, 

 if at all. 



be due to this direct action of the greater mass of animals. He 

 rightly thought there should be a quantitative determination of the 

 salt content at the end of the experiment. 



Drzewina and Bohn (192 1&, 1928) replied by calling attention to 

 the small size of the Convoluta worms, which are about 3 mm. long. 

 They calculate that the amount of salt such worms would carry 

 would not affect sensibly the salt concentration of the solution, but 

 have not refuted this criticism by experiment. 



At Woods Hole I have had an opportunity to test this situation 

 with two main objectives: First, the question of fact involved: Is 

 there a greater protection furnished by the presence of large num- 

 bers of animals in hypotonic sea-water when compared with fewer 

 similar animals in the same volume of water? And, second: What 

 is the mechanism of the protective action of the group, if it be found? 



For these studies a turbellarian, Procerodes wheatlandi (Girard), 

 Figure 16, was selected as representing a form taxonomically some- 



