Chap. XVIII.] THE HYDRA. 339 



an inverted test-tube in sunlight and testing the bubble of gas 

 collected. 



Besides chlorophyll corpuscles, the endoderm cells may contain 

 clear spaces (A., vac.), and sometimes irregular masses, which 

 resemble food particles. If this be so, we have in these cells a 

 process of intracellular digestion, solid food-particles being taken 

 into the cell just as we find to be the case in such a single-cell 

 organism as amoeba. And in this connection it is interesting to 

 note that these cells are capable of altering their form and 

 pushing forth pseudopodial processes (A., ps.). If a hydra be 

 teased up fresh, some of the endoderm cells may be seen in 

 amcebiform motion. Occasionally thread-cells may be found in 

 the endoderm. It may be that, as has been suggested, these 

 have been taken in with the food and ingested. 



The body- cavity, which lies within the endoderm cells, is 

 subject to much variation of diameter. It extends from near 

 the foot to the mouth, and sends prolongations into all the 

 tentacles. It is not readily seen in green hydra (the outer clear 

 space in Fig. 99 representing ectoderm and not the whole body- 

 wall), but is more easily observed in the rarer reddish polyp, 

 Hydra fusca. 



Processes of Multiplication. (1.) Budding. When well fed 

 the hydra multiplies by a process of budding. The bud appears 

 first as a little green knob, then becomes pear-shaped, and ere 

 long develops mouth and tentacles, the mouth appearing some- 

 what suddenly. It thus reaches a size which may fully equal 

 half that of the parent, after which the connection between the 

 parental body-cavity is obliterated by the ingrowth of ectoderm, 

 and the daughter hydra separates to lead a free existence. Such 

 a bud in an advanced stage is shown in Fig. 99, A. Sometimes 

 there may be several buds in various stages of development, 

 when the hydra has the appearance of a compound organism. 

 This process is wholly asexual. 



(2.) fission. It is said that hydra may also multiply by 

 fission, a constriction appearing in the middle of the body, and 

 separation of the organism into two parts occurring at this 



