HYDRA. 127 



swept into the cavity by the ciliary action of the internal 

 cells. Sometimes animals as large as water-fleas {Daphnia) 

 are caught, but in this the Hydra makes a mistake. So simple 

 is the Hydra that a cut-off fragment containing samples of the 

 various kinds of cells in the body, and not too minute, may 

 grow into an entire animal. Thus the Hydra may be artifici- 

 ally multiplied by being cut in pieces. It is not true, however, 

 that when turned inside out (a delicate operation), ectoderm 

 becomes endoderm, and idee versa ; the two layers retain their 

 characteristics, and the animal gradually rights itself 



General Structure. — The body is tubular, and the six to 

 ten tentacles are continuous with its cavity. Of the two 

 layers of cells, ectoderm and endoderm, the outer is trans- 

 parent, the inner contains abundant pigment. On the 

 tentacles especially, one can see, even with the low power of 

 the microscope, numerous clumps of clear stinging-cells. 

 The male cells are formed in several bulgings of the ectoderm, 

 a short distance below the bases of the tentacles ; a single 

 ovum is borne in a larger bulging further down ; but the male 

 and female organs usually occur on different individuals, or 

 on the same animal at different times. In favourable nutritive 

 conditions, the Hydra bears daughter buds, which are event- 

 ually set adrift. 



Minute Structure. — The outer-layer or ectoderm includes 

 several different kinds of cells : — 



(i.) Ordinary covering cells. 



(2.) Muscular cells, with contractile basal processes which 

 lie on the " middle lamina," between ectoderm and 

 endoderm. There is no precise warrant for calling 

 these cells " neuro-muscular." 



(3.) Stinging-cells, nematocysts, or cnidoblasts, within each 

 of which there lies a long eversible thread or lasso 

 (cnidocil) bathed in poison. 



(4.) Ganglionic cells, with many fine connections, especi- 

 ally with the stinging-cells. 



(5.) Interstitial cells, which fill up chinks, and seem to 

 grow into reproductive cells. 



(6.) Glandular cells on the basal disc. 



The inner-layer or endoderm is less varied in structure. 

 Its cells are pigmented and ciliated, but in the absorption of 

 food-particles active amoeboid outflowings are seen. As the 



