142 



THE CELL 



the Rhizopoda eagerly seize upon them, so that after a short time 

 their whole bodies are quite filled with them. 



Infusoria (Fig. 50) eat Flagellata, unicellular Algae and Bacteria, 

 conveying them into their endoplasm through an opening in their 

 cuticle which functions as a mouth. Here a vacuole filled with 

 fluid forms itself round each foreign body, which undergoes 

 digestion. 



FIG. 60. ActinospTicerium Eichliorni (after R. Hertwig, Zool., Fig. 117): M medullary 

 substance with nuclei (n) ; B cortical substance with contractile vacuoles (CD) ; Na 

 nutrient material. 



In a similar manner to that shown by unicellular organisms, 

 many tissue cells of Metazoa devour solid substances offered to 

 them, and digest them. 



Intracellular digestion, as it has been termed by Metchnikoff 

 (V. 12), occurs very frequently in Invertebrates ; it may be best 

 demonstrated by means of feeding experiments with easily recog- 

 nisable substances, such as granules of colouring matter, globules 

 of milk, spores of fungi, etc. In some Ccelenterata the ectoderm 

 as well as the endoderm takes up foreign bodies. The tentacular 

 ends of Actinia may load themselves with carmine granules, which 



