200 



PHYLUM CCELENTERA 



mesenteries are long, ciliated, and non-glandular ; they 

 are respiratory in function and cause an upward current, 

 that in the sulcus being downward. Many Alcyonarians 

 are dimorphic, having in addition to the typical polyps 

 {autozooids) dwarf siphonozooids , with suppressed tentacles, 

 strongly developed sulcus, no mesenteric filaments, and 

 often ill-developed mesenteries. Their function is to 

 drive currents of water through the canal systems of 

 the colony, and they are sometimes reproductive as well. 

 With the exception of one small family of solitary forms 



Fig. io6.— Alcyonarian spicules. 



(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), iDut they usually pass into the mesogloea. 

 The nematocysts are usually small. A number of Alcyon- 

 arians are viviparous ; the embryo is usually a planula. 



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

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

 forms a spreading network or fiat plate, e.g. Clavularia, a type of 

 Stolonifera (Fig. 104, 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, 

 e.g. Xenia and Alcyonmm, types of Alcyonacea (Fig. 104, II.). 



