190 



ZOOLOGY 



SECT. 



formed of discharged nematocysts. The simplest form of skeleton 

 is found in the solitary Alcyonarian genus Hartea (Fig. 131 ), already 

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

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

 spicuh.ir skeleton occurs in the " Dead-men's fingers " (Alcyonium. 

 Fig. 140). where spicules of varying form are found distributed 

 throughout the mesoderm of the ccenosarc. In TuUpora (Fig. 135). 

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

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



PH.. 140. Alcyonium palmatum, A, entire colony ; B, spicules (After Cuvier.) 



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 

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

 Red Coral of commerce (CoraUiii.m, Fig. 132) 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 ccenosarc. 

 Another type of skeleton is found in the Antipatharia (Fig. 137) 

 and in the Gorgonacea (Fig. 141). It also consists of an axial rod. 

 extending all through the colon}- and branching with it, but is 



