20 ON POLYPS. 



are visible. In a short time a deposit of calcareous matter takes 

 place, which cicatrizes the opening, the marks of which however can 

 be traced for a considerable period, until at length the increase of 

 this secretion continuing with the growth of the animal, entirely 

 obliterates all appearance of its having existed. 



In the earliest period of its developement, the foot-stalk by 

 which the young is united to the parent, as well as its radiating 

 disc, is entirely enveloped with the soft parts of the animal ; but as 

 the upper portion spreads, and assumes its characteristic form, the 

 pedicle is left naked, and the gelatinous coating extends only to the 

 line where the separation afterwards takes place. 



(24.) It is generally supposed that the calcareous matter which 

 forms the skeleton of these madrepores is perfectly external to the liv- 

 ing crust which secretes it, and accordingly is absolutely inorganic, 

 and removed from the future influence of the animal which produced 

 it. Such a supposition appears, however, at variance with the facts 

 above stated, and incompatible with many circumstances connected 

 with the history of the lithophytous polyps. On trying to detach the 

 soft envelope from the surface of the skeleton, the firmness of their 

 adherence would render such a want of connexion improbable, they 

 appear to be, as it were, incorporated with each other ; and besides, 

 the separation of the fungia from the peduncle which joined it to 

 its parent during its earlier growth, necessarily supposes a power of 

 removing the calcareous particles after their deposition. It is 

 therefore almost demonstrable that the earthy matter secreted 

 by the polyp is deposited in the tissue of its substance, and 

 still remains, in a greater or less degree, subject to absorption 

 and removal : of this, however, we shall have fuller evidence 

 hereafter. 



(25.) It is astonishing how nearly the animal and vegetable king- 

 doms approximate each other in the lower orders of these calcareous 

 zoophytes. Admitting the animal nature of fungia, we find calcareous 

 skeletons, essentially similar in their chemical composition, produced 

 by a large tribe of organic forms, long classed with the creatures we 

 are now considering, which modern observations have clearly shown 

 to be of vegetable nature.* 



These are the Corallines, (Linn.) which, although so nearly re- 

 sembling the skeletons of polyps, that Cuvier, Lamarck, and others, 

 scrupled not to admit them into the animal circle, have been proved 



* Schweigger, Anatomische Physiologische Untersuchungen liber Corallen. Berlin, 



