152 Papers from the Marine Biological Laboratory at Tortugas. 



Diadema. A number of specimens of Hipponoe were examined for pro- 

 tozoan parasites without results. 



METHODS. 



In comparing the physiological nature of the forms in question certain 

 characters were chosen which could be expressed quantitatively and the 

 animals examined with respect to them. Those selected all had to do with 

 the length of time the organisms could maintain their normal activities under 

 various conditions differing from those to which they were accustomed. 

 The method adopted was to place the animals under the new conditions and 

 note the time that elapsed until all visible movements ceased. In the case of 

 ciliates this time corresponds very closely with that at which irreparable 

 injury, resulting in death, has been done to the cell, and therefore furnishes 

 a more or less accurate criterion of the general vitality of the organisms. 



In the present experiments the number of individuals studied was 

 always large and the range of variation within the species small, and there- 

 fore it was found possible to choose, as the point for comparison, the time 

 at which all the individuals of the given species had been rendered motion- 

 less. This point was easy to determine and the general agreement in the 

 results obtained on successive days by the use of the above method showed 

 it to be sufficiently accurate in a case, such as the present one, where it was 

 desired to express the relative and not the absolute resistance of the four 

 forms in question. Most of the experiments were repeated a number of 

 times with material obtained from different sources, so as to eliminate acci- 

 dental errors. The characters, in respect to which the different forms were 

 compared, were as follows: (i) Ability to live outside the body of the host; 

 (2) ability to live in the body-fluid of other related animals; (3) length of 

 life of the parasites after the death of the host; (4) resistance to CO2; 

 (5) resistance to H2S; (6) resistance to the decomposing proteid substances; 

 (7) resistance to H2SO4; (8) resistance to KOH. 



EXPERIMENTS. 



ABILITY OF THE PARASITES TO LIVE OUTSIDE THE BODY OF THEIR HOST. 



It is well known that protozoan parasites show a varying ability to live 

 apart from their host. In general, the less specialized the parasite and the 

 looser the relation between it and its host, the longer it may be expected to 

 live under conditions other than the natural ones. The forms in question 

 are not highly specialized and, as might be supposed, are for the most part 

 able to live for a considerable time outside the intestine of Diadema, though 

 so far as these experiments go, a continued existence of this sort is in no case 

 possible. The point of particular importance, however, from the standpoint 

 of the present investigation, is that the four forms studied show marked 

 differences in behavior when removed from the body of their host. If a 

 number of food pellets from the intestine of a Diadema be shaken up with a 

 little fresh sea-water, none of the parasites obtained in this way is at first 



