140 PHYLUM C&LENTERA. 



internally, and, by growing over the out-turned endoderm, 

 from the lips downwards, restores the normal state. 



In favourable nutritive conditions, the Hydra forms buds, 

 and on these a second generation of buds may be developed. 

 A check to nutrition or some other influence causes the 

 buds to be set adrift. Besides this asexual mode of 

 multiplication, the usual sexual reproduction occurs. 



General structure. --The tubular body consists of two 

 layers of cells, i.e. the animal is diploblastic. The cavity 

 is the gut, and it is continued into the hollow tentacles. 

 These, when fully extended, may be longer than the body. 

 The mouth is slightly raised on a disc or hypostome. Of 

 the two layers of cells, the outer or ectoderm is transparent, 

 the inner or endoderm usually contains abundant pigment. 

 On the tentacles especially, even with low power, one can 

 see numerous clumps of clear stinging cells. The male 

 organs appear as ectodermic protuberances a short distance 

 below the bases of the tentacles ; the ovary, with a single 

 ovum, is a larger bulging farther down. Both male and 

 female organs may occur on the same animal, either at one 

 time or at different times, but often they occur on different 

 individuals. The buds have the same structure as the 

 parent body, but in origin they appear to be wholly ecto- 

 dermic. 



Minute structure. The outer layer or ectoderm includes the 

 following different kinds of cells : 



(1) Large covering or epithelial cells, within or between some of 

 which lie the stinging cells. The epithelial cells are somewhat conical, 

 broader externally than internally, and in the interspaces lie interstitial 

 cells. By certain methods, a thin shred can be peeled off the external 

 surface of the ectoderm cells. This is a cuticle, i.e. a pellicle no longer 

 living, produced by the underlying cells. 



(irt) Many of these large cells have contractile basal processes, or 

 roots, running parallel to the long axis of the body, and lying on a 

 middle lamina which separates ectoderm from endoderm (Fig. 65, E). 

 The cells themselves are contractile, but there is special contractility 

 in the roots. Like the muscle cells of higher animals, they contract 

 under certain stimuli, and are often called " neuro-muscular." But the 

 discovery of special nerve cells (Jickeli) shows that even in Hydra 

 there is a differentiation of the two functions of contractility and 

 irritability. 



(2) Small stinging cells or cnidoblasts occur abundantly on the upper 

 parts of the body, especially on the tentacles. Each contains a pro- 

 trusible nematocyst. This consists of a sac, the neck of which is 



