6 THE AMERICAN MONTHLY [Jannaiy, 



flows on and leaves the refuse behind. At times of seizure the activity of the 

 flow is greatl}^ increased, and there is no more wonderful sight than that of 

 an amoeba engaged in surrounding its prey. It seems to be almost conscious, 

 so deftly does it complete its work. In the Heliozoa. however, the change 

 of form is mainly confined to the protrusion or retraction of the rays, the body 

 mass remaining quite constant. In the shelled protoplasts it is still further 

 restricted by the chitinous or siliceous test, so that the pseudopods are put 

 forth from one definite part of the whole, the so-called mouth or opening in 

 the shell. But even then these processes never remain the same long. They 

 increase in length or ai-e retracted ; they vary in width or in direction, becom- 

 ing angular instead of straight, or curved, or twisted on themselves. A retic- 

 ular structure is said to have been demonstrated in the protoplasm of Litha- 

 mcsba, and Lankester says it is not improbable that a reticular difterentiation 

 of the general protoplasm similar to that of the nucleus may be found to exist 

 in all cells. 



Turning now to the contractile vacuole, the first point is to distinguish it 

 from other vacuoles, permanent or gastric. The permanent vacuoles contain 

 water or special chemical secretions of the protoplasm, such as oil drops or de- 

 posits of solid consistence. The gastric vacuoles are due to the taking in of 

 water with the solid particles of food. But the contractile vacuole is very dif- 

 ferent in character and function. ' It may be seen to burst,' says the author 

 just quoted, 'breaking the surface of the protozoan and discharging its liquid 

 contents to the exterior ; its walls, formed of undifferentiated protoplasm, then 

 collapse and fuse. After a short interval it re-forms by slow accumulation of 

 liquid at the same or a neighboring spot in the protoplasm. The liquid is 

 separated at this point by an active process taking place in the protoplasm 

 which probably is of an excretory nature, the separated water carrying with 

 it nitrogenous waste-products. A similar active formation of vacuoles con- 

 taining fluid is observed in a few instances {Arcella^ some Amcebas) when the 

 protoplasm separates a gas instead of liquid, and the gas vacuole so produced 

 appears to serve a hydrostatic function.'* 



One more point remains to be considered in regard to the life-history of 

 the Rhizopods, namely reproduction. Concerning this little is positively 

 known. Division is the most common mode, and this may be in a greater 

 or less degree. We may have two parts, as in Amoeba, or many, as in Ar- 

 cella. Spores, amoeboid, or flagellate, have also been observed. Carpenter 

 cites Prof. Edwards as authority for such ' swarm-spores,' which swim about 

 like infusoria, in the case of Amoeba. f Leidy \ says that ' from the researches 

 of Mr. Carter it would appear that in Amoeba and Euglypha, i-epresentatives 

 of the Lobose and Pilose Protoplasts, the endosarc becomes resolved into 

 nucleated cells, which are of the nature of ova, while the nucleus is I'esolved 

 into granuliferous, non-nucleated cells, finally breaking up into their constitu- 

 ent granules, which are of the nature of spermatozoids.' Leidy figures Ar- 

 cella in pairs and surmises these to be cases of conjugation ; but I am inclined 

 to believe that some of these are rather instances of the formation, by division, 

 of new individuals from the parent cells ; for it is noteworthy that the in- 

 dividuals difter in size, in color, and in the character of the shell, some having 

 almost none at all. However, one of the authors in the J//c. Diet, asserts 

 positively that he has seen Arcella in conjugation. Leidy figures also a number 

 of nearly colorless individuals of Arcella, which I believe to be young forms 

 in process of development, the shell not yet having been secreted or being of 

 very delicate texture. In this view I am supported by the writer in the Mic. 

 Diet, just quoted. He adds that the shell is cast sevei^al times before arriving 



* ^ftcy. Brii., Frotozoa. ^ The Microscope. t Rhizopods. 



