728 THE ANIMAL KINGDOM. 



support in this region. A horny axial skeleton is found as a simple rod 

 in Pennatulidae, or as a branching rod in Psendaxonia and Axifera where, 

 however, it is attached by a basal expansion to some foreign object. It is 

 lamellate and then may consist of horny and calcareous layers alternating ; 

 or be divided into successive joints of alternately horny and calcareous 

 nature, e. g. in Isis ; or it may contain cavities filled with a spongy or 

 calcareous mass ; or finally be impregnated throughout with calcareous 

 matter. Its centre is sometimes hollow, filled with a spongy material or 

 secondary calcareous deposits. In some instances, e. g. Sclerogorgia, the 

 horny joints viMelithea and Mopsea, the axial skeleton appears to be com- 

 posed of spicules with horny sheaths which fuse together, whilst the hard 

 calcareous joints of the two last-named genera consist almost exclusively of 

 coherent spicules. Corallium has an entirely calcareous axial skeleton which 

 is formed by the fusion of calcareous spicules at first separate. The horny 

 or partially horny, partially calcareous axis of the Axifera differs from that 

 of the Pseudaxonia in being covered by a superficial epithelium which is 

 invaginated basal ectoderm. An epithelium also covers the surface of the 

 axis in Pennatulidae but its origin is uncertain \ The horny sheaths of 

 the spicules, like the spicules themselves, are derived from mesoglaeal 

 cells which have in the first instance an ectodermic origin ; and the cal- 

 careous cementing material which unites the spicules of Corallium is 

 probably similarly derived 2 . The calcareous skeleton of Heliopora is 

 peculiar, and is composed of tubes large and small the adjacent walls 

 of which appear to fuse. The large tubes form calycles for the autozooids 



1 The axial skeleton of Pennatulidae is contained within a septum which separates a dorsal 

 from a ventral axial chamber, both in communication with the siphonozooids. The septum in 

 question is produced, according to Wilson, in Renilla by a growth of endoderm cells from the 

 aboral extremity of the primitive zooid. It consists of an axial and two superficial layers of cells 

 continuous at the growing edge of the septum. Smooth oval calcareous spicules appear in the axial 

 cells at an earlier date than the acicular spicules of the deep ectoderm cells. An adult Renilla has 

 no axial skeleton, but Wilson's observation suggests that the epithelium covering the axial rod of 

 other Pennatulids and the rod itself are of endodermic origin. 



The dorsal and ventral chambers of the peduncle in Renilla are lined by endoderm, beneath 

 which is a layer of longitudinal and circular endodermal muscles. Longitudinal muscles occur also 

 in the septum. 



See W T ilson, Ph. Tr. 174, 1884, pp. 765-771, and pp. 776-782. 



2 The facts detailed in this account of the skeleton are chiefly taken from von Koch. The 

 skeletal elements may be classified as follows : 



(1) Exoskeletal : horny cuticle of Clavularia, &c. ; axial skeleton of Axifera t in which 

 the skeletal epithelium is in continuity with the basal ectoderm. 



(2) Mesoskeletal, (a) of ectodermic origin ; spicules ; their horny sheaths ; axial skeleton of 

 Psetidaxonia where no epithelium is present ; tubes of Tubipora ; calcareous axis of Corallium ; 

 calcareous lamellate skeleton of Heliopora ; 



() of endodermic origin : axial rod of Pennatulidae. 



It is a mistake to consider the spicules, &c. as exoskeletal because the cells which give origin to 

 them are of ectodermic descent. The fact that the cells are detached and wander into the mesoglaea 

 deprives them of their ectodermic character. 



