ALCYONARIAN CORALS 



105 



tion in the colony, afford one of the principal characters for 

 the recognition of genera and species (Fig. 45). 



In many cases the spicules cease to grow when they have 

 reached a certain size and remain free from one another in 

 the soft tissues, so that when the colonies die and the soft 

 tissues are dissipated the spicules are distributed. But in 

 others {e.g. Corallium and Tubipora) the spicules grow until 

 the}' come into contact with one another and become tightly 

 packed together. In this way a skeletal structure persists 

 after death which represents the general form of the colony. 



The genus Heliopora 

 stands by itself as the 

 only recent Alcyonarian 

 that forms a continuous 

 calcareous skeleton with- 

 out spicule formation after 

 the manner of the Madre- 

 poraria. 



In another group of 

 Alcyonaria which may be 

 called the Gorgonians, the 

 substance Keratin, closely 

 allied to horn, enters into 

 the composition of the 

 skeletal structures. In 

 Gorgonia itself, an axis is 



formed of pure keratin,. and this supports a thin crust or 

 bark consisting of the polyps, with their connecting tissues 

 and the calcareous spicules. On the death of the colony 

 the bark is dissolved and washed away by the sea, the 

 horny axis alone remaining intact. 



In some Gorgonians the horny axis is impregnated with 

 calcium carbonate, and in others the axis consists of alternate 

 horny nodes and calcareous internodes. 



There are a few Gorgonians which consist of a long 

 unbranched stem attached by a disc-shaped expansion at 

 the base to a foreign substance, but usualty the main stem 

 divides into secondary branches, and these ramify again 

 and again before they terminate in numerous delicate free 



9 V-** oXK"^?^* 



^r^ T 



D 



c 



Fig. 43. — Spicules of Alcyonaria. A, 

 Melitodes : B, Isis ; C, Gorgonia ; D, 

 Echinomuricea. 



