IV PHYLUM CCELENTERATA 195 



always possess an ectodermal calcareous skeleton. This order 

 includes the vast majority of .Stony Corals (Figs. 146 and 156). 



Order 3. — Axtipatharia. 



Compound, tree-like Zoantharia in ■which the tentacles and 

 mesenteries are comparatively few (6 — 24) in number. A skeleton 

 is present in the form of a branched chitinoid axis, developed from 

 the ectoderm, which extends throughout the eolonv. This ordei- 

 includes the " Black Corals " (Fig. 150). 



Sub-Class II. — Alcyonaria. 



Actinozoa in which the tentacles and mesenteries are always 

 eight in number. The tentacles are pinnate, i.e. produced into 

 symmetrical branchlets. There is never more than one siphono- 

 glyphe, which is ventral in position, i.e. faces the proximal end of 

 the colony. The mesenteries are not arranged in couples, and 

 their longitudinal muscles are all directed ventrally, i.e. towards the 

 same side as the siphonoglyphe. 



Order 4. — Alcyonacea. 



Alcyonaria in which the skeleton usually consists of calcareous 

 spicules ov small irregular bodies occurring in the mesoglcea, but 

 probably originating from wandering ectoderm cells. The common 

 " Dead' men's fingers " {Aley ,nium. Fig. 153) has a skeleton of this 

 type. In some cases the spicules become aggregated so as to pro- 

 duce a coherent skeleton, which may form a branched axis to the 

 whole colony, as in the jjrecious Red Coral (Corallium, Fig. 145), 

 or a series of connected tubes for the individual polypes, as in 

 the Organ-pipe Coral (Tnhiporu, Fig. 148). In the " Blue Coral " 

 (Heliopora) the skeleton is a massive structure resembling that of 

 the Madreporaria. Most genera are compound; a few, such as 

 Hartea — which, however, is probably a larval form (Fig. 144) — ■ 

 are simple. 



Order 5. — Gorgoxacea. 



Compound tree-like Alcyonaria, with a calcareous or horny 

 skeleton of ectodermal origin forming a branched axis throughout 

 the colony. Spicules are present in the mesoglcea. There is no 

 siphonoglyphe. The beautiful " Sea-fans " belong to this group 

 (Fig. J 54). 



Order 6. — Pennatxjlacea. 



Alcyonaria in which the colony is usually elongated, and has 

 one end embedded in the mud at the sea-bottom, while the 

 opposite or distal end bears the polypes, usually on lateral 



