STRUCTURE OF HYDRA 



177 



rise to a white Hydra, which is supposed to imply that the partner 

 Algse do not migrate into the egg when there is no hght. In the other 

 species of Hydra, the pigment is quite different from chlorophyll. 

 The active lashing of the flagella causes currents which waft food in 

 and waste out. If some small animal, stung by the tentacles, is thus 

 wafted in, it may be directly engulfed by the amoeboid processes of 

 some of the cells, and it has been noticed that the same cell may be at 



— ^■-..^■ 







End 





/TI 



JtC 



-«^^^^-;; 



B 



Fig. 88. — Minute structure of Hydra.— Ki\.QX T. J. Parker and Jickeli. 



A. Ect., Ectoderm ; mg., mesogloeal plate ; %i.c., stinging cell ; End., endo- 



derm with flagella and amoeboid processes. 



B. M.C., Nerve cell, and st.c, stinging cell. 



C. Stinging cell with ejected thread ; n., nucleus. 



D. Mesogloeal plate (mg.) with contractile roots resting on it. 



E. m.c. Muscular cell with contractile roots, c.r. 



one time flagellate and at another time amoeboid (cf. the cell-cycle, 

 Fig. 59). After this direct absorption the food is digested within the 

 cells, and while some of the dark granules seen in those cells may be 

 decomposed pigment bodies, others seem to be particles of indigestible 

 debris. Thus Hydra illustrates what is called intracellular digestion, 

 such as occurs in Sponges, some other Coelentera, and some simple 

 " worms." But experiments show that some of the protein of the 

 food may be digested in the gut cavity, and subsequently absorbed. 

 Thus it seems that both intracellular and extracellular digestion occur. 



12 



