ON POLYPS. 19 



matter, dipping down into the interstices of the plates, and cover- 

 ing the whole frame-work. In the figure, the darker portion indi- 

 cates the living crust ; whilst from the lighter parts it has been re- 

 moved, to show the stony skeleton itself. There are no arms or 

 moving parts adapted to the prehension of food, and no separation 

 of organs adapted to the performance of the vital functions has 

 hitherto been described ; the thin membranous film apparently 

 absorbs the materials of its support from the water of the 

 ocean, and deposits within its substance the calcareous par- 

 ticles which it secretes, moulding them into the form peculiar 

 to its skeleton, which it gradually enlarges as its own extent 

 increases. 



(22.) The gelatinous investment, however, gives certain dubious 

 indications of vitality, and possesses the power of contracting itself 

 so as to retire between the laminae of its skeleton when roughly 

 handled, and thus conceal itself from injury. Upon the surface of 

 the soft crust are seen a number of vesicles indicated in the figure, 

 which were regarded formerly as rudimentary tentacula, from the 

 circumstance of their being able to contract and vary their dimen- 

 sions ; recent observations however lead to the belief that they are 

 cavities filled with air, and serving an important purpose in the eco- 

 nomy of the creature, namely, that of preventing it from being 

 turned upside down by the occasional agitation of the ocean, as in 

 such case the animal has been found by experiment to have no power 

 of restoring itself to its former position, and consequently perishes : 

 these air-vessels may therefore be looked upon as floats, which, ren- 

 dering the upper surface more buoyant than the inferior, materially 

 assist in preventing such an accident ; for, as it lies quite loose and 

 unattached upon the surface of the sand, it is subject to be lifted 

 up from its bed by any sudden roll of the sea, and deposited at a 

 considerable distance from its former place. 



(23.) The reproduction of fungise is effected by the developement 

 of sprouts or gemmae, which pullulate from the animal substance as 

 buds issue from a plant, and remain for some time fixed to 

 the parent by a species of foot-stalk, which sustains them until 

 they have attained to a considerable size ; the young fungiae being up- 

 wards of an inch in diameter before they become detached. When 

 mature, they separate from the top of the stony peduncle which hi- 

 therto supported them ; and at this time, the skeleton of the young 

 fungia, when divested of its fleshy part, shows a circular opening 

 beneath, through which the radiating plates of the upper surface 



