SKELETONS, SHELTERS AND SPECIAL DEFENCES 637 



crystals of calcite are deposited. In the blue coral Heliopora the skeleton 

 consists of crystalline fibres of aragonite fused into lamellae. In the horny 

 corals or gorgonians the axial skeleton consists of a horn-like material, 

 gorgonin, arranged in branched and lamellated fashion. Calcareous matter 

 sometimes occurs in the axis. Overlying the axial skeleton is a system of 

 calcareous spicules and plates of various shapes. In the red coral Corallium 

 gorgonin is absent and the skeleton consists of a solid axis of calcareous 

 spicules cemented together by calcium carbonate (125). 



Gorgonin is a protein which, from X-ray diffraction evidence, shows 

 affinities with collagen {Balticind). The sulphur content is lower than in 

 keratin; high tyrosine content is characteristic; cystine residue is higher 

 than in gelatin. In addition, monoiodotyrosine, diiodotyrosine and 

 bromine analogues appear to be constituent amino-acids of the skeletal 

 protein (p. 73) (81). 



A few analyses of alcyonarian corals are listed in Table 15.1. The con- 

 tribution of organic matter to the skeleton shows much variation. It is very 

 low in Corallium (0*06 %), about 1-6 % in blue coral Heliopora, 2-3 % in the 

 organ-pipe coral Tubipora, 29 % in the sea-pen Pennatula and 52 % in the 

 sea-fan Gorgonia. The axis of the latter is almost entirely organic matter 

 (about 95 %). The chief mineral constituent of Heliopora is CaC0 3 (99 %). 

 In skeletons of other alcyonarians, CaC0 3 accounts for 80-92% of mineral 

 matter. Content of MgC0 3 is high, from 6-17 %. Species from warm waters 

 contain proportionately much more magnesia than those from cold north- 

 ern or cold deep waters (21, 23). 



Madreporaria (true or stony corals) are mostly colonial in habit. They 

 are provided with a hard calcareous exoskeleton, which is secreted by the 

 epidermis and lies wholly outside the polyp body. The skeleton of each 

 polyp is a cup-shaped structure containing vertical ridges radiating from 

 the centre to the periphery. The skeleton of a whole colony is made up of 

 the conjoined skeletons secreted by each polyp. In the solitary corals, 

 Fungia, Caryophyllia, Balanophyllia, the skeletal cup of each polyp is large, 

 0-5-25 cm in diameter. Most corals are colonial, with small or minute 

 polyps, 1-30 mm in diameter. The colonies vary from flat to spherical in 

 shape, and consist predominantly of CaC0 3 , of which only the surface is 

 occupied by living substance. 



Madreporarian corals have been repeatedly analysed. Organic material 

 forms up to 7% of dry matter; most of this probably resides in encrusting 

 material or polyps. The skeleton is almost entirely CaC0 3 in the form of 

 aragonite (21, 23, 58). 



Skeletal material is continually being secreted by the bases of the polyps 

 and the colony increases in length and diameter. By the formation of hori- 

 zontal plates in the cups the polyps are pushed upward and so retain their 

 position on the surface of the mass. 



Annelids. Annelids are essentially soft-bodied animals. Some, like the 

 Aphroditidae, are covered with scales or dense felt-works of chaetae. The 

 chaetae of Aphrodite consist of chitin and aromatically tanned protein (in 



