132 SYNOPTIC COLLECTION. 



of corals living to-day, but at present so much uncertainty 

 exists in regard to their true relationships that we consider 

 them as ancestors of the whole group of Madreporaria. 



The skeleton of the Madreporaria is not spicular, like 

 that of the Alcyonaria, but it is a hard, solid secretion of 

 carbonate of lime. The theca in the Imperforata is 

 not pierced by openings or pores, so that there is no 

 system of canals connecting different corallites. In the 

 Madreporaria Fungidae the young are imperforate and the 

 adult perforate, while in the Perforata there are numerous 

 pores both in the young and the adult stages, and the 

 corallites are in communication with one another. 



The structure of these three groups is essentially similar 

 to that of the sea anemones or Hexactiniae just described, 

 with the exception that the Madreporaria possess a skele- 

 ton. The formation of the skeleton has been studied by 

 many, notably by von Koch 1 and Ogilvie.' 2 The larva 

 begins to form a skeleton after it is attached. The first 

 rudiment is a disc perforated in the center which after- 

 ward becomes entire. This disc is between the ectoderm 

 of the larva and the rock to which the latter is attached, 

 and is a secretion of the ectoderm. It cannot be formed 

 by either the endoderm or the mesoderm, since it is sepa- 

 rated from these layers by the ectoderm. In time there 

 appear radial ridges or upward foldings of the basal disc. 

 The ectoderm in these foldings secretes carbonate of lime 

 and thus limy septa are formed. In one genus observed 3 

 there were twelve mesenteries and twelve septa, six in the 

 chambers between each biradial pair of mesenteries 

 (entosepta) and six others in the chambers between two 

 adjoining biradial pairs (exosepta). In the younger zoons 

 there were twelve mesenteries and only six septa, and 



1 Mitth. d. Zool. Stat. Neapel, III, 1882. Morph. Jahrb., V, VI, 

 VIII. See also Fowler, Quart. Journ. Micr. Sci., XXV, 1885. 



2 Proc. Roy. Soc. London, LIX, 1895; Phil. Trans. Roy. Soc. 

 London, B, CLXXXVII, 1896. 



3 Madreporia variabilis. 



