Elementary Zoology! 



supporting lamella, which is simply an excretion of the cells 

 which border it, as is the cuticle of the earthworm an excretion 

 of the epidermis. 



It will be noticed, therefore, that the hydra's body is built 

 up of two distinct layers of cells which are different in character 

 and in function ; the outer layers provide the sensitive muscular 

 and protective elements, the internal the 

 digestive. The distinctness of these two 

 layers may be impressed upon the mind 

 of the student by relating a curious 

 chapter, or rather paragraph, in the history 

 of error, which concerns this animal. An 

 ingenious naturalist of the last century 

 succeeded in inducing hydras to swallow 

 a worm attached to a thread, and then 

 pulling on the thread when the worm was 

 fairly swallowed, turned the creature 

 inside out. He asserted that this reversal 

 made no manner of difference to the 

 animal, who thereupon used its outer 

 coat as a stomach, and its stomach as 

 a covering. But a still more ingenious 

 Japanese naturalist confirmed, it is true, 

 the statement that a hydra could be with ease turned inside 

 out ; but found also that when left to itself the hydra quietly 

 reversed matters, and assumed its original condition. The 

 cells, in fact, are specialized to perform their several parts, 

 and could not play any other. 



Fig. 8.— Thread cells of 



Hydra. Highly magnified. 



The thread is everted in the 



left-hand figure. 



