DIFFERENTIAL INHIBITION IN OBELIA 475 



resorption would be accelerated and the differential inhibition 

 made more specific. Apart from their plentifulness, Obelia 

 and Campanularia are suitable material because : 



(i) The zooids are conveniently far apart and attached to 

 the hydrocaulus by a fairly long stem ; 



(ii) The zooids in their natural condition are highly differen- 

 tiated structures compared with the rest of the colony ; 



(iii) The stages of resorption can be conveniently determined 

 by reference to the hydrotheca. 



On the other hand there is the disadvantage that it has 

 poor viability, which means that resorption takes place even 

 in the controls in clean sea-water. This, however, occurs 

 long after it has done so in the toxic solutions. 



Care was taken to ensure that the colonies were clean and 

 healthy and free from Diatoms and Protozoa, and that all the 

 polyps were normal and fully extended. 



The experimental solutions are referred to in terms of 

 concentrations of KCN ; but since the solvent was sea and 



N 

 not pure water such an expression as ^n does not give us the 



actual ionic concentration. 



The solutions were made up in shallow glass dishes, in each of 

 which a small number of stems of Obelia each bearing eight 

 zooids (Table I) or one zooid (Tables II to IV) were placed. 

 In every case the material used was fresh from the sea. KCN 

 solutions were kept covered owing to the volatile nature of 

 KCN, and changed every day. 



Eesorption can be divided into five stages (Text-fig. 1). 

 Text-fig. 1, a, represents a normal zooid (also PI. 26, fig. 1). 



First Stage. The tentacles are first affected. They may 

 become apposed to the hypostome, and may shrink so 

 as to become shorter than the hypostome. Adjacent 

 tentacles may fuse (PI. 26, fig. 9), indicating an interesting 

 change in consistency. The mouth closes, but the hypo- 

 stome is still prominent. Ciliary action continues in the 

 enteron as in the normal zooid (Text-fig. 1, h, and PI. 26, 

 figs. 2 and 3). 



