Chap. IX.] 



SPICULES OF ANTHOZOA. 



281 



non-spiculate Anthozoa take up foreign bodies into, and 

 thereby strengthen, their ectoderm. 



The next stage iaseen in Alcyoniurn, where definitely 

 formed but scattered spicules are found in the layers 

 of connective tissue. Where the skeleton becomes 

 continuous it may be horny, and where, as in Gorgonia, 

 a number of polyps are connected together, the skele- 

 ton of the common trunk forms a horny axis ; in the 

 mesodermal tissues of the polyp spicules with an. 



B 



Fig. 116. -A, Triaxial spiculeof Calcisponge (Ascetta Wanca) ; B, Simple 

 Acerate Spicnle of Reniera ; c, Six-rayed Spicule of the Hexactinel- 

 lidge. 



organic basis are developed, which, on the death of the 

 animal, merely form a crust on the axis. In Isis the 

 axis is calcined at certain points only ; so that it is 

 alternately horny and calcareous. In the red coral the 

 whole of the axis is calcined (Fig. 117). In other 

 cases, as in the organ-pipe coral, the hard deposits are 

 laid down in the wall only of the polyp (Fig. 118 A), 

 and these tubes become connected with their neigh- 

 bour by lateral outgrowths, and so form a continuous 

 hard mass. In others, as in the only "coral" found 

 on our own shores (Caryophyllia) the deposits in 

 spicules is not confined to the wall, but extends 

 also into the septa (Fig. 118 B) ; in others, as in 



