168 FRESH-WATEE RHIZOPODS OF NORTH AMERICA. 



Auerbach* states that he has found in a single Arcella as many as 

 forty nuclei. Claparfede and Lachmannf remark that individuals are fre- 

 quently found with a single nucleus ; but it is not rare to see as many as 

 twelve or fifteen. Hertwig and Lesser J confirm the statements of these 

 observers, and state that under favorable circumstances they observed a very 

 variable number of nuclei, mostly more than five. Carter § says that there 

 are but two nuclei, one on each side of the sarcode mass, and this accords 

 with my own observations. Usually I have been able to detect two nuclei, 

 occasionally one only, and rarely three. Perhaps, however, in some of the 

 larger specimens, what I have casually assumed to be food-balls or contrac- 

 tile vesicles, may have been in part rather of the nature of nuclei. 



In the usual views of Arcella from above or from beneath, two or three 

 to half a dozen or more contractile vesicles are seen along the circular 

 border of the sarcode mass, some slowly enlarging, one or more collapsing, 

 and several reappearing. The Arcellas feed on various algse and decaying 

 vegetal matters, but I have not been able to determine whether they 

 feed on infusoria. They are found in all standing waters, in the superficial 

 sediment, and also on the surface of submerged plants. A favorite resort 

 is the under surface of floating leaves of aquatic plants. 



A singular condition not unfrequently occurring in Arcella is the pro- 

 duction of one or more conspicuous air-bubbles within the sarcode mass. 

 These I have observed gradually to enlarge, and then slowly to diminish 

 and disappear without the evident escape of anything. In a single indi- 

 vidual I have observed as many as six bubbles at once. They appear to 

 be due to the development of gas, and their disappearance to its absorption. 

 From experiments of ButschH, he suspects the gas to be carbonic dioxide.|| 



Olaparfede and Lachmann declare that Arcella changes its shell several 

 times during the course of its life. When the animal becomes too large 

 for the shell, it constructs a new one. In this condition, the Arcella almost 

 entirely passes out of its old shell, and at the aperture forms a large mass, 

 which secretes upon its surface a new shell. In such a case, two shells 

 are observed applied to one another, mouth to mouth, the one thick and 

 dark, the other delicate, and at first perfectly colorless, but later pale 



*Zeitschrift f. wissens. Zoologie, vii, 425. 

 tfitudes surles Infusoires et los Khizopodes, i, 444. 

 tAroliiv f. mik. Anatomie, x, Sup. 97. 

 §An. Mag. Nat. Hist, xviii, 1864, 221. 

 II Archiv f. mik. Anat. xi, 1875, 459. 



