240 ^ ^ The JJnitij of the Organism 



digesting, as is the case in the Reticularia. Consistency in 

 descriptive treatment and clear thinking demand that the 

 sarcode processes of those protozoans in which the struc- 

 tures are wholly or even chiefly nutritive in function should 

 no longer be called pseudopodia. Some such tenn as tro- 

 pJwrhiza ought to be applied to them, especially where, as 

 in the Keticularia, they are digestive. Whetlier or not 

 the structures "need explanation on the basis of organs and 

 tissues,'' they certainly need description and definition that 

 shall set forth their true nature. Fortunately considerable 

 study has been devoted to them and the rest of the peri- 

 pheral sarcode in the Rhizopods, and to the extra-capsular 

 sarcode of the Radiolaria, so wc already know much about 

 the facts. 



Calkins ^ has well summarized the information we possess 

 concerning the "pseudopodia" of Sarcodina. From this 

 knowledge we are able to say with the greatest assurance 

 that these creatures lead their lives — maintain their locus 

 in space, whether of fixity or movement, respond to external 

 stimuli, procure, ingest, digest, and assimilate their food, 

 solid and gaseous, and propagate their kind — no less defi- 

 nitely- and hardly less variedly than the larger multicellular 

 animals. All these things they do through the instrumen- 

 tality of definite and definable anatomical elements; and I 

 would insist that we can iustify the refusal to call these 

 elements organs and tissues because they occur within the 

 limits of single cells only by having first so defined organ 

 and tissue as to exclude from them all organic elements not 

 composed of cells. 



The Unjustifiahle Conception that Unicellular Organisms 



Can Have No Tissues 



As a matter of fact this illogical course is exactly the 

 one that is widely followed. "A tissue is, therefore, a com- 



