ORGANIC AND INORGANIC MATTER COMPARED. 75 



escaping observation. Here we have an ex- 

 ample of a free, independent, gelatinous polype, 

 of the simplest structure, the digestive appa- 

 ratus being a simple excavation in its sub- 

 stance ; it is in fact merely a digestive sacculus, 

 the orifice of -which is fringed around with 

 prehensile tentacles. Now suppose one of these 

 animals should greatly expand, and deposit 

 within itself a calcareous substance, acting as 

 a sort of rude skeleton, and hence become, 

 though not fixed, incapable of locomotion — such 

 a being we have in the Fmigia, one of the Ma- 

 drephyllioea. In this creature the stony axis 

 has the upper surface adorned with radiating 

 plates, rendering it not unlike a mushroom. 

 The species of fungia are found in the Indian 

 seas, and lie loose and unattached upon the soft 

 sand at the bottom of the water. The gela- 

 tinous investment is contractile, and repro- 

 duction takes place by buds or gemmules, 

 which at length become detached, and are car- 

 ried away by the waves." 



Let us advance another step — let us picture 

 to ourselves a gelatinous extension common to 

 many polypes, united as it were into one com- 

 pound unity, and secreting for the internal 

 support of the whole, a branching calcareous 

 tree, with pits or cells on the branches, in 

 which the polypes individually reside, and from 

 which they may protrude and expand their 

 tentacles. Examples of this form we see in 

 many madrepores, as Oculina, Dentipora, etc. 

 In other instances, the gelatinous expansion may 



