08 NINETEENTH REPORT. 



special preparation. From the opening of the shell extend long, slender 

 pseudopodia. The protoplasm is attached at base and dome in a wavy 

 line. 



NORMAL BEHAVIOR. 



Physiologically, Jrcella is quite amoeboid, witliin the restrictions im- 

 posed by the shell and other factors. It progresses by the same pseudo- 

 l)odial action, and is similar in food relations. Due to the mechanical 

 impediment of tlie shell, it camiot advance witii the effective rolling 

 motion of the larger amoebas, but must laboriously extend its pseudo[)()- 

 dia, contract its protoplasm, and drag its shell forward. It creeps about 

 the leaves of water plants and on the bottom debris of shallow, sheltered 

 water. In a culture jar, its method of locomotion is evident, but progress 

 is exceedingly slow. 



Generally several long, clear pseudopodia are extended to one side. 

 They are clear and transparent except for ingested food particles, and are 

 normally simple, but may be branched. Now and then an individual is 

 seen to extend a long slender pseudopod and wave it slowly back and 

 forth. Some authors have interpreted this as "feeling for food," but to 

 Calkins (10), it suggests the origin of flagellar structures. 



In its food and feeding habits, the present genus is not markedly differ- 

 ent from the better described Amoeba. The pseudopodia are so long and 

 slender that it is confined for food to the ooze of decaying vegetation, 

 and to certain algae. Although a considerable part of tlie body pro- 

 toplasm may be extruded from the shell, the latter is an effective hin- 

 drance to engulfing large and vigorous Infusorians. 



If a food particle is encountered small enough to be manipulated by 

 the thread-like pseudopodia, it is ingested and passes slowly to the body 

 mass, where digestion is completed. If it is too large to be disposed of 

 in this way, other pseudopodia may be projected and the whole body 

 drawn up to a point over the particle. Within the body the nutriment 

 is absorbed and the indigestible particles egested. Tliis naturally occurs 

 at the mouth of the shell, so that there is a localization of egestion slightly 

 in advance of the amoeba. Contractile vacuoles are numerous and active. 

 There is no evidence that any particular selection of food occurs. There 

 is, however, a degree of discrimination indicated by tlie fact that sand 

 particles and other indigestible materials are not ingested to any great 

 extent. 



In cultures rich with algae, the Arcellas creep about and become filled 

 with the smaller organisms in normal fashion. An astonishing phenom- 

 enon, however, is the ingestion of a filamentous alga twenty or thirty 

 times as long as the diameter of the shell. How it is possible for the 

 animal to coil the long strand within its shell is a j^robleni, but the fact 



