276 



CHAPTER XVIII. 



ZOA NTH A R I A— continued. 



SECTION II. MADREPORARIA RUGOSA (TETRACORALLA). 



The corallum in the Madreporaria Rugosa may be simple or com- 

 posite, and is composed of compact, solid sclerenchyma, the theca 

 being complete and imperforate. The septa are usually well devel- 

 oped and lamellar, with smooth or dentated edges ; but they are 

 sometimes rudimentary. The symmetry of the corallum is almost 

 always obviously bilateral ; and the septa are generally of two orders, 

 alternately long and short. The septa are, typically, developed 

 according to a tetrameral system, new septa being produced along 

 one median and two lateral lines, while a fourth line is commonly 

 marked by a long or short septum or a fossula. A well-marked septal 

 " fossula " is usually present, and generally corresponds with a much 

 reduced primary septum, but there may be three or four fossulae. 

 Endothecal tissue in the form of dissepiments is usually largely 

 developed ; and very generally the dissepiments unite with one 

 another in the central portion of the visceral chamber so as to 

 form well-marked "tabulae." The mode of increase in the com- 

 posite coralla is mostly by lateral or calicular budding, and a true 

 ccenenchyma is wanting. 



It has been supposed by some naturalists that the so-called 

 Rugose corals might possibly be referable to the Hydrozoa, and, 

 at any rate, that they are not truly Madreporarian. On the other 

 hand, Mr Quelch has recently abolished the Rugosa as a distinct 

 division, and has united the corals formerly placed therein with the 

 Madreporaria Aporosa. Some of the forms included in the old 

 order of the Rugosa may, as previously seen, be referred without 

 difficulty to the Aporosa, and as regards most of the others the view 

 advocated by Mr Quelch can be supported by evidence of no small 

 weight. The resemblances, namely, between the corallum of the 

 typical Rugosa and that of the Madreporaria Aporosa are so numer- 



