160 PHYLUM CCELENTERA. 



bear vertical ridges or costae, and these may be connected 

 with neighbouring costae of other polypes by horizontal 

 shelves or dissepiments. Both septa and costae correspond 

 to intermesenteric spaces. (See Shipley's Zoology of the 

 Invertebrata^ pp. 68-71.) 



ALCYONARIA. 



In the Alcyonarian polype there are always eight pinnate 

 tentacles and eight mesenteries attached to the stomodaeum 

 or gullet. There is one longitudinal ciliated groove 

 (siphonoglyph or sulcus) in the stomodaeum. The 

 mesenteries bear retractor muscles, all situated on the 

 sulcar aspect (see Fig. 74), and each mesentery bears a 

 mesenterial filament. Many Alcyonarians are dimorphic, 

 having in addition to the typical polypes (autozooids) dwarf 

 siphonozooids, with stunted or suppressed tentacles and ill- 

 developed mesenteries. Their function is to drive currents 

 of water through the canal systems of the colony. With 

 the exception of one small family of solitary forms (Haimeidae), 

 the Alcyonarians form colonies which are in various ways 

 supported by spicules or by spicules and an axis. The 

 spicules, which take the most diverse forms, seem to be 

 begun at least by ectodermic cells (a pair to each spicule), 

 but they usually pass into the mesogloea. A number of 

 Alcyonarians are viviparous ; the embryo is usually a 

 planula. 



Colonies are formed in different ways, (i) A parent polype gives 

 off hollow stolons or solenia, which bud off new polypes, and the whole 

 forms a spreading network or flat plate, e.g. Clavulan'a, a type of 

 Stolonifera (Fig. 76, I.). 



(2) The polypes may be crowded together so as to form bundles 

 raised on a stalk, or lobose fleshy growths with the polypes projecting 

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

 Xenia and Alcyonium, types of Alcyonacea (Fig. 76, II.). 



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

 common upright ccenenchyma, in which the polypes are imbedded, 

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

 cemented spicules, e.g. Coralliutn, a type of Pseudaxonia. 



(4) Or the vertical extension of the colony may be effected by 

 a horny secretion from the polypes, which comes to form an axis, 

 really outside of the polypes though encrusted by them. This axis may 

 be purely horny or in part calcareous, e.g. Gorgonia and Isidelta, types 

 of Axifera (Fig. 76, III.). 



(5) Fifthly, the vertical extension may be due to a great elongation 



