6 LOWER INVERTEBRATES. 



There is also a curious pulsating vesicle within the endosarc, but often it encroaches 

 upon the ectosarc so much as to seem a part of the latter. This vesicle originates as a 

 clear spot in the protoplasm, which slowly enlarges imtil it reaches a considerable size, 

 when it suddenly collapses. There is a regularity in this occurrence which may be 

 observed to be repeated several times in a minute. The function of the contractile 

 vacuole, which is ^•ery common among the Infusoria, has not yet been fully determined. 

 It is supposed that it subserves the respiratory process, but some authors regard it as 

 subserving an excretory purpose. Whether the fluid of the vacuole is forced out into 

 the surrounding water as the vesicle closes has not been satisfactorily demonstrated, 

 although there is strong evidence pointing to that conclusion. 



Food is taken into the body of the Amoeba through any part of the surface. A 

 portion of the ectosarc extends around the prey, enclosing it along with some of the 

 water, which then sinks down into the endosarc, where it forms a so-called food-ball ; 

 such food-balls may become quite numerous in a single animal. Ehrenberg, supposing 

 them to be permanent stomachs, gave the name Polygastrica to those Protozoa in which 

 he observed them. Any portion of the Amceba's body will serve the purpose of a tem- 

 porary stomach, in which food may be digested and assimilated. The indigestible por- 

 tions are ejected at any part of the surface, but usually at the posterior part, near the 

 contractile vesicle. The food of the Amoeba is usually of a vegetable nature. The deli- 

 cate filamentous desmids seem to be a favorite food, and diatom remains are often found 

 in the Amoeba in great abundance. In assimilating 

 the nutriment of a filamentous desmid, the Amoeba 

 passes along the filament, enveloping cell after cell, 

 seemingly passing the plant directly through its body, 

 absorbing the contents and rejecting the indigestible 

 portions. 



The Amoebse propagate by division and perhaps 

 by a process of conjugation ; at least an appearance 

 of conjugation has been observed in a few cases. 

 When the circumstances of life are unfavorable, the 

 AmoebiE may become encysted, by which means they 

 <5j-^r,.,j.^ .<,,, — ^iua«isB«*^- are able to withstand great changes of external con- 

 t;. „ o T^j, ■ 7 , IT 1 A ditions. When about to become encvsted, the re- 



Fia.S. — Diffiugtaurceolala. Enlarged. .... 



mains of food and particles of indigestible matter are 

 rejected, and the animal assumes a spherical shape precisely like Protomyxa, Fig. 1, a, 

 soon becoming surrounded by a more or less thick membrane eomjjosed of several layers. 

 In this protected condition the Amoeba may rest a long time, and then, by ruj^turing its 

 • envelope it may again come forth apjjarently unchanged. But in some cases a change 

 takes place within the capsule which results in the formation of a large number of 

 spherical germs or spores, each of which probably escapes and grows into a new form, 

 as in the case of J^rotomt/xa, already described. 



A large number of the Lobose protoplasts are ]3rovided with shells, many of them 

 of regular and beautiful form. Of these, Difflugia, Fig. 3, may be taken as a repre- 

 sentative genus. The shell of this Rhizopod is spherical or oval, composed of grains 

 of sand mingled with frustules of diatoms, spicules, etc., cemented together. The 

 sarcode-body almost fills the shell, and is attached to it by protoplasmic threads passing 

 to the fundus and sides. The shell is o]3en at one end, where the blunt cylindrical 

 pseudopodia are projected either for the prehension of food or as organs of locomotion. 



