THE SUB-KINGDOM CCELENTERATA. 



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of the inner layer are more closely applied to one 

 another, their size is somewhat larger, and their 

 contents more transparent, than are the same parts 

 of the outer layer. The chief difference between 

 the layers is, however, in mode of increase, the 

 ectoderm growing from within outwards, the endo- 

 derm from without inwards. 



Even in Hydra, the lowest of Coelenterate or- 

 ganisms, these two primitive layers are readily 

 observable. But this animal exhibits, at certain 

 seasons of the year, a tendency to break up into 

 particles of a sarcode aspect, which retain for a 

 long time an independent vitality. Nor are such 

 amoeboid masses wanting in the tissues of higher 

 Coelenterata. The significance of such facts should 

 not escape our notice, since, at least, they serve to 

 indicate the nature of the foundations on which 

 the house of life has been constructed. 



But the body-substance of the Coelenterata by 

 no means always presents that simple structure of 

 its layers which the above expressions might seem 

 to imply. Vacuolation and fibrillation here like- 

 wise perform their part, and the latter metamor- 

 phosis is often carried to such an extent as to give 

 rise to true muscular fibres, though these are not, 

 as in the higher animals, accompanied, in most 

 cases, by an evident nervous system. Thread-cells, 

 already described, the body-layers elaborate, as 

 also reproductive elements, pigment-masses, and 

 those other granular structures, which seem adapted 

 for secretion. By conversion and excretion, outer 

 growths are formed which serve either for support, 

 ornament, or protection ; while, by deposition of 

 calcareous salts, the beautiful internal skeletons 

 known as " corals " become variously produced. 



