COELENTERATA 73 
side, the ventral, there is a special groove (Fig. 51) lined with 
long cilia, the siphonoglyph. 
The stomodaeum is supported by eight mesenteries ; on the 
ventral side of each of these are placed the powerful retractor 
muscles, which draw the polyp swiftly into the corallite at the 
approach of danger. The two dorsal mesenteries alone are 
continued far below the lower edge of the stomodaeum in the 
form of mesenteric filaments, their edge being much thickened, 
bilobed, and covered with cilia. The endodermal cells are 
probably some of them amoeboid, and digestion may be intra- 
cellular. 
The ova are found attached to the dorsal and dorsolateral 
mesenteries, immediately below the stomodaeum. 
The tabulae are formed by the mesogloea splitting into 
two layers: the outer remains attached to the ectoderm, the 
inner layer with the endoderm; the latter shrinks away from 
the outer layer, and then in this contracted condition begins to 
secrete a fresh layer of spicules, which ultimately stretches 
across the corallite. Hence the space below each tabula is 
morphologically a space in the mesogloea. : 
The polyps of the Zubipora remain free and distinct one 
from another ; in other groups of the OcractTINIA, however, they 
may be sunk in a well-developed coenosarc, as in Alcyonium, 
commonly known as “dead men’s fingers,’ or they may be 
arranged side by side, their lateral surfaces fusing in the form of 
a leaf-like plate, as in the Pennatulidae (Fig. 52). In Aleyoniuwm 
the skeleton is not continuous, but consists of spicules scattered 
loosely through the coenosare. The leaf-like plates of Penna- 
tula are borne on each side of a rachis, this is continued into a 
stalk free of polyps. Both stalk and rachis are traversed and 
supported by a long calcified horny rod, secreted by an 
epithelium whose origin is uncertain. 
Among the Alcyonidae and Pennatulidae the individual 
zooids are often of two kinds. In Pennatula, for instance, the 
leaf-like expansions are composed of a single layer of polyps 
(autozoids) fused side by side, whilst the zooids (siphonozords) 
cover that surface of the rachis on to which the bases of 
the leaves do not extend, and pass up between the leaves. 
The zooids differ from the polyps, having no tentacles or 
