ELEMENTARY VITAL PHENOMENA 143 



into the lungs passes readily through the thin walls, to be eagerly 

 sucked up by the red blood-corpuscles and transported throughout 

 the whole body. 



Dissolved substances also always bathe the surface of the cells. 

 In the plant they ascend along with the water in fine tube-like 

 canals and thus are brought directly to the cells. In the compound 

 animal body some of the cells, such as those of the intestinal 

 epithelium, are in immediate contact with the dissolved food-stuffs 

 of the intestinal tract, while all the other tissue-cells are bathed 

 by the blood-current, which brings to them the dissolved food in a 

 definitely elaborated form. In such invertebrate animals also as 

 possess no proper blood-circulatory system, the cells either stand 

 in immediate contact with the surrounding water or are supplied 

 with juices that bathe the cells in fine intercellular spaces. The 

 simplest relations, finally, exist in unicellular organisms, such as 

 Algcv, Bacteria, and others, which live constantly in a nutrient 

 solution, either in water containing salts or in organic liquids. 



The ingestion of solid food occurs in only a few cell-forms. 

 Among unicellular organisms all Eliizopoda, most ciliate Infusoria, 

 and some flagellate Infusoria, take in solid food. In the complex 

 cell-community this power is possessed by the leucocytes or white 

 blood-corpuscles, which, therefore, have been termed by Metschni- 

 koff phagocytes (eating cells), by amoeboid wandering cells, 

 which play in the lower animals the role of leucocytes, by amoeboid 

 egg-cells, such as occur in sponges, and by the intestinal epithelium- 

 cells. Among these forms of cells two types may be distinguished, 

 according to the manner of ingestion of solid food. The one type is 

 able to take the food-masses into its living substance at any 

 desired point upon its surface such are all amoeboid cells, to which 

 belong Khizopoda, leucocytes and intestinal epithelium-cells ; the 

 other type possesses a special, constant mouth-opening such are 

 the ciliate and the flagellate Infusoria, which have a definitely fixed 

 body-form with a denser cutaneous layer. All cells, however, that 

 take in solid food are able to do it only by means of active 

 movements of the body. 



The ingestion of food by Amoeba may serve as an example of 

 the first type. The process, which has been observed in full only 

 relatively seldom, takes place somewhat as follows. An Amoeba, 

 which is being observed in a drop of water under the microscope, 

 creeps about the glass slide by letting the living substance of its 

 formless protoplasmic body flow here and there into broad, lobate 

 projections (Fig. 43). Suddenly it turns toward a small alga-cell 

 lying in the vicinity, and creeps on until it touches the cell. Its 

 protoplasm immediately begins to flow around the latter in the form 

 of the usual lobate pseudopodia ; but the cell is shoved away by the 

 encroaching protoplasm and the amoeba is obliged to make a new 

 attempt to surround the cell. After several fruitless attempts it 



