244 WM - A " KEp NER AND JOHN F. BARKER. 



plete quota of nematocysts at its surface. On December 8, 1916, 

 a Microstoma, that contained seven or eight nematocysts and so 

 greatly rilled with food that the pharynx of its posterior zooid 

 was everted, was placed with a Hydra. Within 12 minutes 

 the Microstoma had egested some of its food and had torn off 

 a part of the Hydra's oral end. Two days later a second well- 

 fed specimen, that contained but two nematocysts, was placed 

 with a Hydra. This specimen ate the Hydra within twenty 

 minutes. Table I., shows that of 42 specimens containing none 

 or few nematocysts, 6 (within two minutes after being placed 

 with Hydras) set to work trying to feed upon the polyps, but 

 five of them got severely wounded while the sixth was eaten 

 by the Hydra. Two others were eaten by the Hydra within 

 twenty-four hours. Of the remaining 34 specimens, 9 had eaten 

 Hydras in periods ranging from 2 minutes to 20 minutes ; while 



4 : ' 



20 accepted Hydras in periods ranging from 24 hours} to I hour ; 

 and 3 remained with Hydra either 5 or 2 days before they ac- 

 cepted Hydra. Two specimens remained with Hydra 24 hours 

 and a third 4 days when they were lost or had died without 

 having accepted Hydra. The conspicuous feature of Table I. 

 is that in many cases Hydra was accepted within a period of 

 minutes, many others within a period of hours, while only a 

 few were accepted within a period of days. 



Microstoma WITH MANY NEMATOCYSTS REACTS TO' Hydra 



TARDILY. 



Table II. makes a sharp contrast with Table I. in this respect. 

 For this table shows the reactions of 18 Microstomas, that had 

 either many or a full quota of nematocysts. All, except speci- 

 men 15, were kept from food for a day or more and yet only 

 one of them, specimen 7, reacted to Hydra within 24 hours and 

 this one contained only about one-third of a full quota of nema- 

 tocysts. Specimen 8 remained with Hydra 24 hours when it 

 was lost, without having accepted Hydra. Except for these two 

 specimens, all the others reacted to Hydra not within periods 

 of hours or minutes but of clays and that despite the fact that 

 they had no food for at least a day. Specimens 17 and 18 were 

 kept away from food in each case four days. They lived four 



