154 CCELENTERA. 



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 mesogloea. 



Taking as a basis the hard parts only, corals may be classified in 

 various ways : 



According to Composition 



(i.) Discontinuous calcareous spicules Alcyonium, &c. ; 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)} " Plorny" and calcareous, e.g., axis of his. 

 (c) Wholly calcareous, in the great majority. 

 According to extent of the hard parts 

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



(2.) Paused in an external tube, e.g., 7\ibipora (Organ pipe coral). 

 (3.) Fused in an axis, e.g., Corallium rubrum (Red coral). 

 (4.) Invading the outer wall (theca), the base, and forming cal- 

 careous septa between the mesenteries, and often, also, a 

 centra] pillar (columella), e.g., massive reef building corals. 

 The terms Sclerodermic and Sclerobasic were formerly much used in 

 the description of corals. The former denoted corals in which the hard 

 parts are laid down by the individual polypes themselves, and support 

 their soft tissues, as in Tubipora, Fungia, and numerous others ; the latter 

 was used in describing cases, like the Red Coral, the Sea Pens, &c., 

 where there is a calcareous skeleton in the connecting substance of the 

 colony. 



According to position of the hard parts 



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

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

 Astrcea, Fungia, Madrepora ; in Gorgonids, Gorgonia and 

 Isis. 



(2. ) " Mesoskeletal," i.e., in the mesoglcea, 

 e.g., spicules of Alcyonium, 



fused spicules of Tubipora, 

 axis of Corallium. 



SYSTEMATIC CLASSIFICATION OF THE COELENTERA. 



The Ccelentera are often classified as follows : 



( fV^npdntfl ( Hydromedusse. 



A. Hydrozoa, I \Siphonophor, 



(^Acraspeda. 

 ( Alcyonaria. 



B. Actinozoa, 



(^Zoantharia. 



C. Ctenophora. 



