12 MARINE BIOLOGICAL LABORATORY. 



which characterizes the Hydra stage. In correlation 

 with this important step, we have one grand and several 

 minor structural features introduced. The digestive cells 

 arrange themselves together in the form of a tubular 

 sack open at one end, thus taking the first step towards 

 a rudimentary alimentary canal. Around this sack, the 

 remaining cells station themselves, forming another sack 

 inclosing the first. The sacks are in close contact, and 

 the walls of the outer one are continuous with those of 

 the inner one at its open end, so that the inner sack may 

 be regarded as an infolding, such as we might rudely 

 ' represent by pushing in the end of a glove-finger. If 

 the material were elastic, so that we could draw out the 

 double wall around the open end into a number of arm- 

 like extensions, we should have a fair model of the Hydra 

 body with its tentacles. 



The cells constituting the inner sack, called the ento- 

 derm, are in the most favorable situation for attending 

 to the food-supply of the entire cell community ; and 

 natural selection has constrained them to specialize in 

 this direction until they have become inoperative in 

 other ways, and even incapable of doing anything else. 

 Trembley succeeded in turning these creatures inside 

 out ; and as they lived on after such treatment, he in- 

 ferred that the functional differentiation of the two lay- 

 ers was so slight that ectoderm and entoderm could 

 exchange places and works. The mistake has only 

 recently been corrected by a Japanese naturalist. Dr. 

 Ischikawa of Tokyo has shown conclusively that Hydra 

 cannot live long turned inside out, and that, if left to 

 itself after the operation, it soon turns itself back into 

 its normal condition. This act of recovery escaped the 



