168 PHYLUM C@LENTERA. 
(ventrally). The mesenteries bear retractor muscles, all 
situated on the sulcar aspect (see Fig. 81), and each 
mesentery bears a mesenterial filament. The two dorsal 
(asulcar) mesenteries are long, ciliated, and non-glandular ; 
they are respiratory in function and cause an upward 
current, that in the sulcus being downward. Many Ale 
cyonarians are dimorphic, having in addition to the typical 
polyps (autozooids) dwarf siphonozooids, with suppressed 
\ 
Fic. 85.—Corallium rubrum, a corner of a colony.— 
After Lacaze-Duthiers. 
A,, Anthocodia or retractile portion of a polyp; 7.2., com- 
pletely retracted polyp, with the verruca or calyx portion 
left protruding ; C., coenenchyma; 7., pinnate tentacles. 
tentacles, strongly developed sulcus, no mesenteric fila- 
ments, 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 
(Haimeide), 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 
