36 E. A. Schäfer, 



water by tlie amoeboid activity of tlie ectoderm cells, but that a pro- 

 cess of iRtracellular digestiou may go oii within tliem, much in the 

 same way as in tlie body of an amoeba. Metsclmikoff') foimd in 

 Ascetta and Halisarca tliat, in tlie species experimented on, the ecto- 

 derm cells did not take in solid particles but tliat they were freely 

 taken in by entoderm and mesoderm cells. This process tlierefore 

 forms almost an exact parallel to tbat whicli occurs in tlie vertebrate 

 embryo as described by KoUmann, and in tlie adnlt vertebrate as I 

 liave liere endeavoured to show. 



In nnmerous other cases amongst the lower Metazoa it lias been 

 proved that both ectodermal cells and also migratory mesodermal 

 cells are directly concerned with the inception of food-particles, which 

 may be submitted to a process of intracellular digestion within them.*) 

 Stül more abimdant evidence has been accumiüating of late years with 

 regard to the intus-susception and digestion of food by entoderm-cells 

 in invertebrate animals, a point of much interest in connection with 

 the subject of absorption in vertebrates, particularly with respect to 

 the manner in wMch undissolved materials of the food may find ad- 

 mission into the columnar cells and the possibility of tlieir undergoing 

 within those cells a furthei' process of digestion and assimilation. I 

 need here only refer briefly to some of the observations that have 

 been made because the facts relative to this subject have already 

 been carefuUy marshalled by others^ especially by Metsclmikoff 

 and Wiedersheim''). The observations, besides those upon Sponges 

 already referred to, have been chiefly made upon Coelenterates 

 and Planarians. Amongst Coelenterates a process of intus-suscep- 

 tion and digestion of food - particles by entoderm - cells has been 

 described in Hydra by Parker*), in several Hydropolyps and Hydro- 

 medusae, in Siphonophora , Pelagia, Ctenophora and Actiniae by 



') Metschnikoff, Untersuchungen ü. d. intracell. Verdauung. Arb. a. d. zool. 

 Inst. d. Universität Wien V, 2, S. 3. 



^) Metschnikoff, loc. cit. 



®) Wiedersheim, Festschrift der 56. Versammlung deutscher Naturforscher. Frei- 

 burg i./Br. 1883. (I regret that I have not been able to see a copy of this memoir.) 



*) T. Jeffrey Parker on the histology of Hydra fusca, in Qnart. Journ. of Micr. 

 Science, 1880. 



