ALCYONARIA. 



169 



each spicule), but they usually pass into the mesoglcea. 

 The nematocysts are usually small. A number of Alcyon- 

 arians are viviparous ; the embryo is usually a planula. 



Colonies are formed in different ways. (l) A parent polyp gives 

 \ off hollow stolons or solenia^ which bud off new polyps, and the whole 

 [ forms a spreading network or flat plate, e.g. 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 

 i on the surface of a dense mesogloeal mass honeycombed by solenia, e.g. 

 \ Xenia and Alcyonium, types of Alcyonacea (Fig. 84, II.). 



(3) Or the colony may raise itself 'in the water by forming 9 



FIG. 86. Alcyonarian spicules. 



common upright ccenenchyma, in which the polyps are embedded, 

 and the medullary part of which may form a substantial axis of 

 cemented spicules, e.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. Gorgonia 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 (Heltolites, etc.). 



