THE OUTER AND INNER TISSUES OF ANIMALS. 285 



says Prof. Huxley, " the higher forms are double animals, 

 and a section of their bodies is, morphologically speaking, like 

 a section of two Hydra, one contained within the other." 

 The relations of the parts may be illustrated thus : Cut off 

 ?.he finger of a leather glove that has a lining ; and let the 

 leather and the lining represent the ectoderm and endoderm 

 of a Hydra. Thrust the point of the glove-finger back into 

 the cavity, until the introverted portion comes out beyond 

 the open end. Cut off the projecting apex of the introverted 

 portion level with the edges of the open end ; and then unite 

 the edges of the introverted portion and the outer portion. 

 The arrangement of structures will then typify that which is 

 common to all animals except the Protozoa and the lower 

 Ccelenterata : the introverted part representing the alimentary 

 canal ; the outer part representing the body- wall ; and the 

 closed cavity between the two representing the peri- visceral 

 sac. This, however, is not the whole parallelism. If in the 

 glove-finger, representing in its original form the Hydra, we 

 suppose the leather standing for the ectoderm to be growing 

 outwards, and the lining standing for the endoderm to be 

 growing inwards, then if in the part that is introverted the 

 same relations of growth are maintained, it is manifest that 

 of its two layers the one which was outermost and is now 

 innermost, will grow towards the open cavity which stands 

 for the alimentary canal, while the other layer will grow 

 towards the closed, cavity standing for the peri- visceral sac. 

 And these are the directions of growth actually found in the 

 parts thus symbolized. 



This simile must not have more meaning given to it than 

 is intended. Though there is reason for suspecting that a 

 re-duplication has taken place in the course of evolution, and 

 that the peri-visceral sac which distinguishes all the higher 

 classes of animals from the lower, has been formed by it ; yet 

 the method of re-duplication cannot have been anything like 

 that described ; and has probably been so different a one as 

 to negative the implied homologies of the layers. The illus- 



