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



drawing together of its margins. The tentacles may be in these cases completely 

 invaginated, as is seen in Corallium and Heliopora. Finally, the (;alyx may be feebly 

 developed, and the whole polyp may withdraw itself into the coenenchyma which 

 completely surrounds the lower portion of the alimentary cavity. When this happens over 

 the entire colony the surface thereof appears smooth and as if covered with small 

 pores. 



A further differentiation of the individual polyps is seen in those cases where the 

 colony consists of heteromorphic polyps, each with diiferent functions. A certain amount 

 of dimorphism is involved in the presence of connecting stolons, so that the tendency to 

 polymorphism cannot be said to be absent even in the simplest forms. Some stolons end 

 blindly and serve simply for the attachment of the colony. Somewhat similar vegetative 

 individuals are represented by the axial polyps of the Pennatulacea and the Holaxonia. 

 Besides the tentacle-bearing individuals or autozooids of Moseley, bud-like individuals may 

 be present without tentacles and with a reduced number of mesenteric folds. These, the 

 siphonozooids of Moseley, are found in very different families — in Sarcopliytum and 

 Lobophytum among the Alcyonidse, in Heliopora among the Helioporidae, in Coralliuvi 

 among the Scleraxonia, in Dasygorgia among the Holaxonia, and among the Pen- 

 natulacea. 



If we attempt to establish a natural arrangement on the lines above indicated, we may 

 first accept the three orders recognised by previous investigators, namely Alcyonacea,' 

 Pennatulacea, Gorgonacea. We may further subdivide the Gorgonacea, or fixed 

 Alcyonaria with colonial axial skeleton, into the two sections Scleraxonia and 

 Holaxonia. The Alcyonacea may form the starting point for the two other suborders. 



ALCYONARIA, Milne-Edwards. 



Polyps and polyp-colonies ; the individual autozooids possess eight pinnate tentacles, 

 and as many uncalcified mesenteric folds 



Order I. ALCYONACEA, Verrill. 



Alcyonacea, Verrill, Proc. Essex Inst., vol. iv. p. 148. 

 Alcyonidx, Milne-Edwards, 1857, Dana, 1859, Kiilliker, 1865. 



Polyps single or in colonies, when the latter, united by endodermic nutritive 

 canals without axial skeletons. 



1 In the Description of Genera and Species the orders are arranged thus — Gorgonacea, Pennatulacea, Alcyonacea. 



