448 SCIENCE PROGRESS. 



small particles of food and digesting them within the cell- 

 substance, just as an amoeba is known to catch and digest 

 its food particles. 



The process of digestion within the cell-substance, or 

 " intracellular digestion," was subsequently discovered by 

 Metschnikoff and other observers to occur in the intestine, 

 or in certain regions of the intestines, of other Invertebrates, 

 and may be said to be now recognised as a normal method 

 of digestion in Ccelentera, in many worms, and probably 

 also in other groups of animals. 



This fact being established, the next questions which 

 arose were : Is this the only method of digestion in Ccelen- 

 tera ? Does no digestion take place in the cavity of 

 the polyps? Is there no extracellular digestion? The 

 first question had to be answered in the negative, and 

 the last two in the affirmative. No one who has seen 

 a Hydra catch and devour a large daphnia can doubt for a 

 moment that some fluid is brought to bear upon its tissues 

 which leads to their rapid disintegration, and to assert that 

 this process occurs within a single endoderm cell, or Plas- 

 modium of fused endoderm cells, would be erroneous and 

 ridiculous. 



The next stage in the history of the investigation was 

 reached when Lankester, in 1881, published an account of 

 his researches on the digestion of Limnocodium and the 

 structure of its endoderm. 



Limnocodium is the name given to the interesting little 

 freshwater medusa which periodically makes its appearance 

 in the Victoria Reoia tank in the Botanical Gardens in 

 Regent's Park. The mouth of this delicate little creature is 

 situated at the end of a quadrangular tube suspended from 

 the centre of the under side of the disc. He found that 

 the endoderm lining the walls of the cavity of this tube or 

 stomach presents three principal forms, which occur in the 

 oral, the mid-gastric and the proximal regions respectively. 



Intracellular digestion was observed in the proximal 

 region only ; small unicellular organisms being enclosed in 

 the substance of the cells, some apparently unaffected and 

 others in various stages of disintegration and dissolution. 



