184 THE INVERTEBRATA 



thickens enormously. In the red coral Corallium rubrum (Fig. 137) 

 there is an upright branched colony with a rigid axis composed ojf 

 spicules compacted together which is the precious coral of commerce. 

 This is clothed by the delicate tissue of the coenosarc from which the 

 short polyps arise and which contains a network of endodermal 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 Mediterranean and the seas 

 of Japan. Dimorphism, as described below for Pennatula, also occurs 

 here. 



Fig. 137. 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 con- 

 fined 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. 138). 



In Pennatula and its relations (suborder Pennatulacea) a single 

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



