26 THE ANTHOZOA 



species which the reader should consult for details of the anatomy and 

 development. 



Although von Koch, some years since, demonstrated the essential 

 difference between the Pseudaxonia and the Gorgonians or true Axifera, 

 many subsequent authors, although they have accepted von Koch's con- 

 clusions, have persisted in bringing the two groups together in the order 

 Gorgonacea. It is evident, from what has been said above, that the 

 Pseudaxonia and the Axifera form two distinct lines of descent, diverging 

 from a common Cornularia-like ancestor, and therefore they must be 

 classed as two distinct branches of the order Syualcyonacea. The sole 

 reason for uniting the two branches in one order is that the higher forms 

 of the two show a remarkable superficial resemblance to one another, a 

 resemblance which is the more remarkable from the parallelism of forms 

 like Melitodes and Isis, both of which, though belonging to widely 

 separate families, have an axial skeleton composed of alternate horny and 

 calcareous segments. The resemblance, striking though it may be on 

 superficial examination, disappears on closer comparison. 



But whilst there is ample justification for keeping the two groups 

 apart, it is not suggested that the line of descent attributed to the Pseud- 

 axonia is beyond criticism. Whilst it is quite possible, and may seem 

 probable, that Leucoella and Solenocaulon are on the direct line of 

 descent of the higher forms of the Pseudaxonia, there is nothing that 

 can be urged against the view put forward by Klunzinger (49) that 

 the Briareidae are descended from forms like the Siphonogorginae, the 

 medullary mass being formed by excessive development of spicules in the 

 partitions separating the stem-canals. The majority of the Pseudaxonia 

 are monomorphic, but dimorphism occurs sporadically in the genera 

 Paragorgia and Corallium. 



ORDER 4. Axifera, von Koch. 



Characters Synalcyonacea, forming colonies consisting of a coenen- 

 chymatous rind investing a horny or calcified axis. The axis may be 

 horny, or composed of a calcified, horny substance, or may consist of 

 alternate segments of calcified and horny substance ; it never contains 

 solenia, and is never formed of fused spicules. The coenenchyme com- 

 pletely invests the axis, and contains solenia and calcareous spicules 

 imbedded in the mesogloea. 



The Axifera (or Gorgonacea) have been the subject of an admirable 

 memoir by G. von Koch (61), to which the reader should refer for 

 morphological and embryological details. The characteristic feature of 

 the group is the axis, which is horny, or consists of a horny basis im- 

 pregnated with salts-of-lime. It is surrounded by a definite epithelium, 

 which is ectodermic, and is derived from the basal ectoderm of the 

 mother zooid of the colony. The mother zooid secretes at its base a 

 horny plate, which lies between the basal ectoderm and the surface of 

 attachment. This is the primordium of the axis. It rapidly increases 

 in thickness, and forms a short column, rounded at the upper end. This 

 column projects upwards into the coelenteron of the mother zooid, 

 carrying before it the three layers, ectoderm, mesogloea, and endoderm. 



