ANTHOZOA ALCYONARIA. 727 



in a well-developed coenosarc, and the fore-part or oral extremity of the 

 body, which is sometimes invaginable, e. g. Corallium, sometimes not so, 

 e. g. Primnoa, is the only region which projects freely. The colony how- 

 ever constituted is either attached or free. When attached the base is 

 extended, if the colony is massive as in the Alcyonidae 1 and Helioporidae^ 

 or relatively small as in the Pseudaxonia and Axifera, the colony of which 

 is a more or less branched and spreading structure, the branches of which 

 are either free or only accidentally fused where in contact, e. g. in Coral- 

 Hum, or united to form a lattice-work. In the latter case they are all 

 disposed in one and the same plane. The colony is free in the Pennatu- 

 lidae. It has an elongated slender axis, the basal portion of which is more 

 or less pointed and sunk in sand or mud. The exposed portion bears the 

 zooids, (i) at its apex ( Umbelluld) ; (2) in a single row (Protocaulidae^ 

 Protoptilidae\ or in numerous and irregular rows along one, the dorsal 

 aspect and the sides, e. g. Funiculina, sometimes however leaving a narrow 

 dorsal streak free ; (3) aggregated on latero-dorsal leaflets as in Pteroeides 

 and Pennatula ; or (4) confined to one aspect of a terminal kidney-shaped 

 expansion as in Renilla. 



The skeletal structures are not less distinctive than the character of the 

 colony. The discontinuous or spicular skeleton is only absent in the non- 

 colonial Monoxenia and the colonial Helioporidae. It is the only skeleton 

 present in the Tubiporidae and Alcyonidae ', the spicules in the first-named 

 being united by minute serratures into a continuous tube for each zooid, 

 except near its oral extremity where they are free. It is present in other 

 Alcyonaria in conjunction with other forms of skeleton mentioned below. 

 The spicules themselves are lens-like, cylindrical, acicular, flattened or 

 stellate, sometimes smooth, or roughened with pointed or warty processes. 

 They are composed of Calcium carbonate with traces of Magnesium 

 carbonate and are tinged with Iron, hence often imparting colour to the 

 colony. The calcite has the typical form of minute rhombohedra ; and in 

 the spicule, layers of rhombohedra alternate with fine layers of an organic 

 substance which is most developed superficially. It is rare for the spicules 

 to project freely beyond the surface of the body as they do in the leaflets 

 of Pteroeides. 



The organic horny skeleton may form either an external sheath or an 

 internal and central branched axis. The first form is a cuticular secretion 

 of the ectoderm, and occurs only in Clavtdaria (Cornularidae\ and Sarco- 

 dictyon, where it is thin near the oral extremities of the zooids but thickens 

 towards their bases. Inasmuch as the spicules of Clavularia near the base 

 may also be coated with horny layers which become connected to one 

 another and with the external sheath as well, the zooids acquire a firm 



1 Alcyoniitm digit atum sometimes occurs in free ball-like colonies. 



