xxxviii THE VOYAGE OF H.M.S. CHALLENGER. 



2. Pleurocorallium, Gray, Proc. Zool. Soc. Lond., 1867, p. 12'3 ; Cat. Lithopliytes 



Brit. Mus., 1870, p. 23 ; Ridley, Proc. Zool. Soc. Lend., 1882. 

 The species of this gemis contain two kinds of spicules, and the polyps are mainly 

 restricted to one surface of the compressed stem and branches. 



Section II. HOLAXOXIA. 



Axifera, v. Koch, Morph. Jahrb., Bd. iv. pp. 474, 476. 



Gorgonacea with an axis which consists of horny material, or of a calcified horny 

 substance, or of alternating joints of amorphous calcareous material and horn. 



The formation of the colonies in the Holaxonia appears to proceed on a somewhat 

 different plan from that observed in the Scleraxonia. For while in the latter the axis first 

 appears as a diflerentiation of the ccenenchyma at the original ba.sal expansion of the 

 colony, in Holaxonia it is formed from the beginning as a central support to the colony, 

 and it is surrounded peripherally by the longitudinal canals. 



The Holaxonia include the following families : — 



V. Dasygorgidae. | IX. Plexauridje. 



VI. Isidse. ! X. Gorgonidaj. 



YII. Primnoidpe. I XI. Gorgonellidte. 



VIIL Muriceidse. i 



The Dasygorgidfe may be regarded as containing the most primitive forms. The 

 polyps are nou-retractile ; the tentacles, which are furnished with spicules, are simply 

 coiled, when at rest, over the oral disc. The axis is at once horny and calcareous. In 

 one direction this family leads on to the Isida; where the axis is differentiated into 

 alternate horny and calcareous portions, and where, at least in some forms, the polyps are 

 like those of the Dasygorgidse. 



From the subdivision Chrysogorgina? of the Dasygorgidse, in which the spicules of 

 the polyps are scale-like, the Primnoidfe may have been derived. In certain forms in 

 which the parts of the polyp bodies are but slightly differentiated, the Muriceidte also 

 suugest an origin from Dasygorgidfe ; though in other forms tlie differentiation of the 

 polvps into a calvcine and a retractile tentacular region becomes more pronounced, until 

 tinallv in the genus Muricea there is a transition to the Plexauridje, where the anterior 

 portion of the polyp may be completely protected within the calyx or within a deep 

 caaienelivraatous cavity. The Gorgonida^ may be regarded as a special division arising 

 from the Muriceidfe or Plexaurid^e. and they finally lead on by the calcification of the 

 axis to the usuallv biradial colonies of Gorgonellid?e. The differentiation of the type 

 uiav be hvpotheticallv indicated in this scheme : — 



