PHYLUM CCELENTERATA 



203 



referred to, in which minute irregular deposits of calcium carbonate, 

 called spicules (sp.), are deposited in the mesogkea. A similar 

 spirillar skeleton occurs in the " Dead-men's finger " ( Alcyonium, 

 Fig. 153), where spicules of varying form are found distributed 

 throughout the mesoglcea of the coenosarc. In Tubipora (Fig. 148), 

 the " Organ-pipe Coral," the mesogloeal spicules become closely 

 fitted together, and form a continuous tube for each polype, the 

 tubes being united by horizontal calcareous platforms (pi.) formed 

 by deposits of spicules in the expansions of the same name already 

 referred to. The skeleton of Tubipora is, therefore, an internal 



Fio. 1..3.— Alcyonium palmatum, A, entire colony ; B, spicules (After Cuvier.) 



skeleton, and in the living state is covered by ectoderm. In the 

 Red Coral of commerce (Corallium, Fig. 145) the originally separate 

 spicules are embedded ' in a cement-like deposit of carbonate of 

 lime,, the result being the production of an extremely hard and 

 dense branched rod, which extends as an axis through the coenosarc. 

 In the Blue Coral (Heliopora), on the other hand, the stony 

 calcareous skeleton is not made up of fused spicules, but is solid 

 from the first. 



Another type of skeleton is found in the Antipatharia (Fig. 150) 

 and in the Gorgonacea (Fig. 154). It also consists of an axial rod, 

 extending all through the colony and branching with it, but is 



