ALCYONARIA, ; 169 
each spicule), but they usually pass into the mesoglcea. 
The nematocysts are usually small. A number of Alcyon- 
arjians are viviparous ; the embryo is usually a planula. 
Colonies are formed’ in different ways. (1) A parent polyp gives 
off hollow stolons or so/enza, which bud off new polyps, and the whole 
forms a spreading network or flat plate, 2g. Clavularia, a type of 
Stolonifera (Fig. 84, I.). 
(2) The polyps may be crowded together so as to form bundles 
raised on a stalk, or lobose fleshy growths with the polyps projecting 
on the surface of a dense mesogloeal mass honeycombed by solenia, 4g. 
Xenia and Alcyonium, types of Alcyonacea (Fig. 84, II.). 
(3) Or the colony may raise itself in the water by forming 2 
Fic. 86,—Alcyonarian spicules. 
common upright coenenchyma, in which the polyps are embedded, 
and the medullary part of which may form a substantial axis of 
cemented spicules, ¢.g. Corallium, a type of Pseudaxonia. 
(4) Or the vertical extension of the colony may be effected by 
a horny secretion from the polyps, which comes to form an axis, 
really outside of the polyps though encrusted by them. This axis may 
be purely horny or in part calcareous, e.g. Gorgonda and Acanella, types 
of Axifera (Fig. 84, III.). 
(5) Fifthly, the vertical extension may be due to a great elongation 
of a single primary polyp which gives off solenia bearing numerous 
secondary polyps, e.g. Pennatula, a type of Stelechotokea (cf. Fig. 
84, IV.). 
An altogether aberrant type is represented by the blue coral 
(Heliopora) and its extinct relatives (Hedoltes, etc... 
