ALCVONARIA GORGONACEA 353 



Sub-Order 2. Axifera. 



The axis in this sub-order may be horny, or horny with a 

 core of calcium carbonate, or composed of horn impregnated with 

 calcium carbonate, or of nodes of horn alternating with internodes 

 of calcium carbonate. It may be distinguished from the axis of 

 the Pseudaxonia by the fact that in no case have definite spicules 

 been observed to take part in its formation. It has been 

 suggested that as the Axifera represent a line of descent distinct 

 from that of the Pseudaxonia they should be placed in a separate 

 order. Apart from the character of the axis, however, the two 

 sub-orders show so many affinities in their general anatomy that 

 it is better to regard the two lines of descent as united within 

 the Gorgonacean limit. It is very improbable that the two 

 groups sprang independently from a stoloniferous ancestor. 



Fam. 1. Isidae. — Tliis family includes all those Axifera 

 in which the axis is composed of alternate nodes of horn and 

 internodes of calcareous substance. 



There can be little doubt of the close affinities of many of 

 the genera of this family with the Melitodidae among the 

 Pseudaxonia. In both the coenenchym is thin and the 

 coelenteric cavities short. No important differences have been 

 observed between the structure of the zooids of the two families, 

 and now that we know that the " nutritive canals " of Melitodes 

 do not perforate the nodes there is no important difference 

 left between the coenosarcal canal systems. The structure and 

 method of calcification of the internodes of the two families are 

 very similar. The main difference between them is that the 

 nodes of the Isidae are purely horny, whereas in the Melitodidae 

 the horny substance of tlie nodes contains calcareous spicules. 



The principal genera are : — Isis, Linnaeus ; Pacific Ocean. 

 This genus forms substantial fan-shaped colonies with, relatively, 

 a thick coenenchym, short stout internodes and black horny 

 nodes. Mopsea, Lamouroux ; Coast of Australia. The verrucae 

 are club-shaped and are arranged in spiral rows round tlie stem. 

 AcancUa, Gray ; principally found in deep water in the Atlantic 

 Ocean but also in the Pacific. The internodes are long and the 

 branches arise from the nodes. ]\Iost of the species occur in 

 deep water, some in very deep water {A. simplex, 1600 to 1700 

 fathoms). In this and the following genera the coenenchym is 

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