CORALS. 141 



With the doubtful exception of the Sea-pens and their allies, in which 

 the axial skeleton is believed by some to be endodermic, the "coral" is 

 due to ectoderm cells which either remain in the ectoderm or wander 

 into the mesoglcea. 



We may arrange "corals" according to different bases of classifica- 

 tion : — 



According to Composition — 



(l.) Discontinuous calcareous spicules — Alcyoniiim, etc; these 



may also occur along with some forms of (2). 

 (2. ) Continuous skeleton. 



(a) Organic and horny, e.g. , axis of many Gorgonids, axis of 



Pennatulids. 

 {i} Horny and calcareous, e.g. , axis of Isis. 

 [c) Wholly calcareous, in the great majority. 



According to extent of the hard parts — 



(l.) Diffuse spicules, e.g., Alcyonium. 



(2.) Fused in an external tube, e.g., Tubi- \ 



/<>ra (Organ-pipe coral). ("Sclerodermic." 



(3. ) Fused in an axis, e.g. , Coralhum 1 



rubruvi (Red coral). ) 



(4.) Invading the outer wall (theca), the , 



base, and between the mesenteries, and \ 



often forming u. central pillar (colu- > " Sclerobasic. " 



mella), e.g., massive reef-building* 



corals. 



According to position of the hard parts — 



(i.) " Exoskeletal," more or less directly continuous with the 

 ectoderm, e.g., in Madrepore corals (reef-builders), like 

 Astrcea, Pungia, Madrepora ; in Gorgonids, Gorgonia 

 and Isis. 

 (2) " Mesoskeletal," 2.1?., in the mesoglcea, 

 e.g., spicules oi Alcyonium, 

 fiised spicules of Tubipora, 

 axis of Corallium. 



Systematic Classification of the Coelenterata. 



The Coelenterata are aquatic and almost wholly marine 

 animals which differ from higher Metazoa in retaining the 

 symmetry of the gastrula, and in having no definite meso- 

 derm nor body-cavity. Associated with the ectoderm there 

 are stinging-cells, and the nervous and muscular cells remain 

 epithelial. The reproductive elements may arise from the 

 ectoderm or from the endoderm. There are two types — 



