42 FRESH-WATER EHIZOPODS OF NORTH AMERICA. 



ferent localities, the nucleus presents considerable range in size. In a num- 

 ber of characteristic individuals it measured about -^th of an inch in width 

 and from one half to two thirds the thickness of the width. In others it 

 ranged from half the size indicated to one fifth greater diameter. 



The nucleus is colorless, homogeneous, indistinctly and finely granu- 

 lar, or more coarsely, uniformly, and distinctly granular. I did not at any 

 time distinguish a distinct membranous wall to the nucleus ; and a distinct 

 nucleolus, if present, escaped my notice. 



In several instances I have seen two nuclei in the same individual, as 

 repi'esented in fig. 2, pi. I. More frequently I have failed to detect a single 

 one, though it is probable that in most cases, if not in all, it was hidden from 

 view by other elements of the endosarc. Occasionally, however, even in large 

 specimens of unusual transparency and unobscured by the presence of food 

 and other materials, as in the individual represented in fig. 7, pi. II, I was 

 unable to discover a nucleus. 



8. Almost invariably there may be seen in the vicinity of the nucleus, 

 and approximating it in size, the clear liquid globule, which constitutes the 

 contractile or pulsating vesicle. It usually occupies a position just back of 

 the nucleus, but not unfrequently, in the movements of the endosarc, is 

 shifted to a position in advance. The contractile vesicle is ordinarily 

 observed very gradually to enlarge, and then rather abruptly to collapse, 

 so as to disappear altogether from view. After a brief interval it again re- 

 appears in the same or nearly the same position. Starting as a minute 

 spherule, the vesicle gradually increases as before, until it reaches its former 

 size, when it once more abruptly collapses. The increase, collapse, dis- 

 appearance, and reappearance of the contractile vesicle occur successively 

 and incessantly in a rhythmical manner, reminding one of the regular action 

 of the heart in higher animals. 



The gradual expansion of the contractile vesicle is slow and more or 

 less variable in the time it takes to reach the full size ; the collapse ordi- 

 narily occupies about a second of time. 



The contractile vesicle appears to be due to the accumulation or concen- 

 tration of water from the surrounding endosarc, which water appears then to 

 be expelled from the bod}- in the collapse of the vesicle. It is perfectly 

 clear, and of a pale, though decided roseate hue, which becomes more pro- 

 nounced with the expansion of the vesicle. The same hue is to be observed 



