ALCYONARIA 



l6l 



thickens enormously. In the red coral Corallium rubrum (Fig. 133) 

 there is an upright branched colony with a rigid axis composed 

 of spicules compacted together which is the precious coral of com- 

 merce. This is clothed by the delicate tissue of the coenosarc from 

 which the short polyps arise and which contains a network of endo- 

 dermal tubes, some of which run along the parallel grooves which are 

 sometimes to be seen on the surface of a piece of precious coral. The 

 mesogloea contains spicule-forming cells derived from the ectoderm, 

 and these travel inwards and add their secretion to the central 

 skeleton. This form occurs at considerable depths in the Mediter- 

 ranean and the seas of Japan. Dimorphism, as described below for 

 Pennatula, also occurs here. 



Fig. 133. Section transverse to the axis of Corallium. After Hickson. 

 A, autozooid; Ax, skeletal axis; S, siphonozooid without tentacles. The 

 ectoderm is indicated by the outer line, the mesogloea by stippling and the 

 endodermal network (solenia) by the irregular spaces in the mesogloea. 



The gorgonians (suborder Gorgonacea) also have upright branching 

 colonies. The supporting axis has, however, an origin, different to 

 the last, being horny and not calcareous and secreted by the ectoderm 

 on what is really the outer surface of the animal. As secretion is 

 confined to an invagination of the basal epithelium which burrows 

 into the whole length of the colony, it appears to be an internal 

 skeleton. The gorgonians are a remarkable feature in shallow tropical 

 seas, forming groves and thickets which challenge comparison with 

 the plant forms of the land (Fig. 134). 



In Pennatula and its relations (suborder Pennatulacea) a single axial 

 polyp grows to a relatively enormous length, sometimes as much as 

 three or four metres, and contains a long horny axis which is possibly 



