176 



COELENTERATA. 



and spreads out so as to form a plate-like expansion of coenenchyma 

 containing endodermal canals from which the new polyps bud 

 (Clavularia rosed). By the increase of this basal coenenchyme we 

 get the massive colonies of Alcyonium, with long polyp tubes and 

 canal system (Fig. 145, D). Another variation in the colonial 

 form may be deduced by supposing the plate-like expansion to be 

 invaginated so as to bring its lower surface to the inside, and the 

 polyps to the outside. Such a process would lead to the production 

 of colonies with an ectodermal axial rod (Gorgonia, Fig. 145, B, 

 ? Pennatula). Calcareous spicules are very generally present. They 

 may occur either in the coenenchyma, or in the walls and tentacles 

 of the polyps, and they may either be loose or coalesced to form a 



B 



FIG. 145. Diagrams to show the budding and mode of formation of the colonies of various 

 Alcyonaria. A, general diagram. B, Gorgonia. C, Tubipora. D, Alcyonium. The enteric 

 space and canals are black. S oesophagus ; se mesenteries ; mf mesenterial filaments ; dh 

 enteron ; sk axial skeletal rods with lines to show the mode of growth. 



continuous corallum. When this coalescence occurs in the polyp- 

 walls we get the thecal tubes of Tubipora; when it occurs in the 

 coenenchyma, the axial rod of the Scleraxonia (Corallium rubrum). 

 Sometimes the spicules are embedded in a horny material, sometimes 

 in a calcareous cement. In the Pennatulacea and the Holaxonia 

 (Gorgonia} there is an axial rod secreted by an epithelial layer 

 contained in the stem or axis of the colony. This epithelial 

 layer is of ectodermal origin in the Holaxonia (Fig. 145, B\ but 

 its origin in the Pennatulacea is doubtful (? endodermal). In all 

 other cases the spicules arise in the jelly of the coenenchyma. 



