378 



ZOOLOGY. 



t> 



of 



lies against and is firmly attached to the mesoglcea. These 

 processes show a faint structure resembling that of 

 unstriped muscle-fibre, and they are distinctly of a muscular 

 nature, their 

 combined con- 

 traction beinj 

 the cause 

 the change of 

 shape of the 

 whole animal. 

 Hence they 

 are called 

 muscle-tails, 

 and the cells 

 which bear 

 them muscle- 

 tail-cells (fig. 

 151, B). 



4. Nema- 

 tocysts. Fill- 

 ing up the 

 spaces between 

 these last are 

 numerous 

 smaller 

 rounded cells, 



the interstitial cells. These may be regarded as reserve- 

 cells, comparatively undifferentiated and ready to take on 

 one of several possible functions as may be necessary. 

 Most commonly, perhaps, they develop into cnidoblasts, cells 

 which secrete within themselves remarkable structures called 

 nematocysts. Imagine a closed bladder drawn out at one 

 end into a long slender hollow thread; let this thread 

 be turned outside in and be coiled up within the main part 

 of the bladder. Imagine further that this bladder is tensely 

 filled with liquid then, evidently, any pressure on the bladder 

 will cause the hollow thread to be more or less completely 

 everted, and a sudden and sufficient pressure would cause 

 the thread to be shot out with considerable violence. Such 



Fig. 194. MEDIAN LONGITUDINAL SECTION OF HYDRA. 

 (Rather diagrammatic.) 



