26 FRESH- WATER ROIZOPODS OF NORTH AMERICA. 



several contractile vesicles situated at the periphery of the body adjacent 

 to the nucleus. In Arcella, the gi-eatest number of contractile vesicles 

 occur, and are ordinarily seen, as the animals are viewed from above or 

 below, along the border of the sarcode mass. 



Food swallowed by the Lobosa and other Rhizopods, and indeed by 

 most of the Protozoa, appears in the endosarc, commonly in spherical balls, 

 mingled with the other constituents. These balls, or rather the spaces 

 occupied by them, have been called vacuoles, and are usually regarded as 

 of temporary character. Ehrenberg supposed them to be actual stomachs, 

 and hence the name of Polygastrica (Gr. polus, many ; gaster, stomach), 

 which he applied to all the Protozoa, including other forms now generally 

 regarded as pertaining to the vegetal kingdom. 



Dr. Wallich considers the so-called vacuoles, or food-vesicles, not in the 

 light of mere spaces, but as temporary vesicles of ectosarc, due to inver- 

 sion of portions of the exterior ectosarc at the time of the inception of the 

 food, or to the contact of water with portions of the endosarc. The food- 

 balls commonly appear with some degree of uniformity in size, and they 

 vary in color and constitution according to the nature of the food and the 

 changes it undergoes during digestion in the endosarc. The solid food in 

 the balls often appears surrounded with a more translucent area than the 

 contiguous endosarc, due to water swallowed with the food Frequently, 

 however, the solid food appears without the translucent area, and in direct 

 contact with the endosarc, in which cases liquid originally ingested with the 

 food and altered in some way has probably been drained off into the con- 

 tiguous endosarc. 



Among the food constituents of the endosarc there frequently occur 

 solid bodies of different forms and extremely different sizes, with or without 

 surrounding liquid, such as diatoms, desmids, fragments of vegetal tissues, 

 and even other Rhizopods included in their shells. 



Associated in like manner with the ordinary food-balls, there are fre- 

 quently to be noticed drops of liquid, clear and colorless, or colored, 

 mingled with the other materials of the endosarc. These particular liquid 

 drops, water-vacuoles or vesicles containing liquid, are due to water swal- 

 lowed in drops, or imbibed and accumulated in drops, or they are the result 

 of more consistent food liquefied in the process of digestion. 



The food appears to be swallowed by the Lobosa, not by a mere 



