18 BULLETIN 120, UNITED STATES NATIONAL MUSEUM. 



to give a clearer understanding of some of the phenomena and rela- 

 tions to be treated, and, second, for the sake of having available a 

 succinct account of this interesting animal. 



During most of the year the Opalinids in the rectum of their host 

 continue to grow, dividing occasionally by longitudinal and trans- 

 verse fission as described. When the reproductive season of the host 

 approaches, that is, in Europe and North America in the spring, the 

 Opalinids begin to divide with greater frequency, multiplication 



r*" 



a 



'^. 



d ^ J 



Fig. 5. FROTOOPALINA INTESTINALIS^ CYSTS : X 673 DIAMETERS. a, AN ENCYSTING INDI- 

 VIDUAL WHOSE NUCLEUS CONTAINS ONE LARGE MACROCHROMATIN SPHERE, 8 MUCH 

 SMALLER MICROCHROMATIN (?) PARTICLES AND NUMEROUS ACHROMATIC GRANULES; 6, 

 AN ENCYSTING INDIVIDUAL WHOSE NUCLEUS CONTAINS BOTH CHROMATIC SPHERES AND 

 ACHROMATIC GRANULES ; C, A CYST SHOWING THREE MACROCHROMATIN SPHERES IN ITS 

 NUCLEUS ; d, AN INDIVIDUAL HATCHING FROM THE CYST, ITS NUCLEUS CONTAINS THREE 

 MACROCHROMATIN SPHERES ; 6, ANOTHER HATCHING INDIVIDUAL WHOSE NUCLEUS CON- 

 TAINS FOUR (two?) MACROCiatOM/XIN SPHERES, A FIFTH (third?) HAVING BEEN EX- 

 TRUDED INTO THE CYTOPLASM ; /, A SIMILAR INDIVIDUAL WHOSE NUCLEUS HAS EXTRUDED 

 TWO MACROCHROMATIN SPHERES AND STILL RETAINS ONE. (After MetCalf, 1909.) 



being more rapid than growth, so that the animals become smaller, 

 until finally they are minute forms, often with but a single nucleus. 

 In the latter cases evidently the division of the body has gotten ahead 

 of the division of the nucleus, so that finally the two nuclei are sepa- 

 rated before they have time to divide. These minute individuals, 

 containing either one or two nuclei, then encyst (fig. 5). (In the 

 multinucleated Cepedeas and Opalinas the encysting individuals may 

 have from 1 to as many as 12 nuclei.) The cysts pass out with the 

 feces of the host into the water to which the hosts have now gone for 



