XIII ALCYONARIA SKELETON — COLOUR 337 



important part iu the building up of skeletal structures in 

 many Alcyonaria. In Clavidaria viridis and in Stereosoma a 

 change in the chemical character of the mesogloea of the body- 

 walls of the polyps leads to the formation of a horny tube, 

 which in the former case is built up of interlacing fibres, and in 

 the latter is formed as a homogeneous sheath. In many of the 

 Alcyonacea which have a compact axial skeleton the spicules 

 are cemented togetlier by a horny matrix. 



In the Gorgonellidae and some others the hard axis is 

 formed of a horny substance impregnated with a crystalline 

 form of calcium carbonate ; but in the Gorgoniidae, many of the 

 Pennatulacea and some other genera very little or no carbonate 

 of lime is found in the horny axis. 



The skeleton of the genus Helio'pora differs from that of all 

 the other Alcyonaria in its development, structure, and form. 

 In the words of Dr. G. C. Bourne,^ " the calcareous skeleton of 

 Hdioi^ora is not formed from spicules developed within cells 

 but is a crystalline structure' formed by crystallisation of car- 

 bonate of lime, probably in the form of aragonite, in an organic 

 matrix produced by the disintegration of cells which I have 

 described as calicoblasts." It is furtlier characterised by its 

 blue colour. A peculiar form of the axial skeleton (Fig. 155), 

 consisting of alternate nodes mainly composed of keratin, and 

 internodes mainly composed of calcium carbonate, is seen in the 

 families Isidae and Melitodidae. In the Melitodidae the nodes 

 contain a considerable number of loose spicules, and the inter- 

 nodes are mainly composed of spicules in close contact but 

 firmly cemented together by a sparse horny matrix. In the 

 Isidae the scanty calcareous substance of the nodes, and the 

 bulk of the substance of the internodes, is formed of amorphous 

 crystalline limestone. 



The Alcyonaria exhibit a great variety of colour. Very little 

 is known at present of the chemistry of the various pigments 

 found in the group, but they may conveniently be arranged in 

 two sections, the soluble pigments and the insoluble pigments. 

 To the former section belong various green and brown pigments 

 found in the anthocodiae and superficial coenenchym of many 

 genera. These are related to chlorophyll, and may be very largely 

 the product, not of the Alcyonarians themselv^es, but of the 



' Quart. Journ. Micr. Set. xli. 1899, p. 52L 

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