ALCYONARIAN CORALS 



121 



are all of one kind, similar to the autozooids of Corallium, 

 and the axis consists of alternate horny nodes and calcareous 

 internodes (Fig. 54). 



This constitution of the axis renders the coral and its 

 branches capable of bending in any direction without break- 

 ing, and is in striking contrast to the 

 axis of Corallium, which is perfectly 

 rigid and can only resist the force of 

 the sea tides by virtue of its solidity 

 and strength. 



There is a passage in the book on 

 Zoophytes by Ellis ^ which is worth 

 quoting here as it expresses remarkably 

 well the meaning of this structure of 

 the axis of Isis. " These joints are an 

 admirable contrivance of Nature to 

 secure the little branches of these 

 animals from being torn to pieces. 

 Without this they could not arrive to 

 the height of which some of them are 

 found, viz., of two or three feet, for 

 by bending freely to and fro with these 

 soft joints they easily resist the violent 

 motions of the sea." 



The colony of an Isis is usually 

 branched in one plane forming a fan- 

 shaped coral, but specimens are some- 

 times found in which the ramification 

 is less regular and an aggregated mass 

 of irregular branches is the result. The 

 terminal branches are thick and end 

 bluntly. 



The calcareous internodes of the main stem may be as 

 much as 10 mm. in length and 10 mm. in diameter and deli- 

 catelv fluted with grooves in which the nutritive canals 

 of the coenenchym lie. The horny nodes, which shrink and 

 become brittle when dry, are about 3-4 mm. in thickness. 



Among the manv genera which are included in the family 



^ The Xatitval History of Zoophvtes, by John Ellis, 1786, p. 103. 



Fig. 54. — Isis hippuns. 

 The axis of one of the 

 terminal branches of a 

 large colony showing the 

 horny nodes and cal- 

 careous internodes. Xat. 

 size. 



