in] HYDRA 53 



(2, Fig. 18). These so-called interstitial cells are young cells, 

 which partly, no doubt, become developed into ordinary ectoderm 

 cells as the older cells die and drop off, and in certain seasons of 

 the year they increase very much in number at certain spots and 

 form the reproductive organs (9 and 10, Fig. 17). The two kinds 

 of organs, male and female, are borne by the same individual ; in 

 the male organ or testis all the cells remain small and become 

 converted into the small spermatozoa; in the female organ or 

 ovary one cell increases very much in size at the expense of the 

 rest and becomes the egg -cell or ovum (10, Fig. 17). There is, 



Fro. 18. Transverse section of Hydra fusca. 



1. Ectoderm cells (myo-epithelial). 2. Interstitial cells. 3. Nematocysts. 

 4. Coeienteron. 5. Endoderm cells. 6. Vacuoles. 7. Food 

 granules. 8. Nuclei. 



however, a third change which these interstitial cells may undergo, 

 which is of the utmost importance to the animal. Some of them 

 move outwards and become wedged between and even embedded in 

 the large ectoderm cells near the surface, each developi< in its 

 interior an oval bag filled with fluid. One end of this bag is 

 turned into the interior of it, forming a long hollow thread. The 

 whole bag is called a thread-capsule or nematocyst (Gr. vy^a, 

 a thread ; KVO-TIS, a bladder) (Fig. 19). If now the cell in which 

 the thread-capsule is situated contracts, since the fluid in the 

 capsule is incompressible, the hollow thread must be quickly turned 



