122 C@LENTERATA. 
the general appearance closely resembles that of a very 
large but short and broad Aydrva. An examination of the 
mouth, however, will show that it is not circular like that 
of Aydra but elongated in one direction, and at each 
corner of the long axis there is a small groove called a 
siphonoglyph. The walls of these grooves are ciliated and 
water apparently passes down one groove and up the other, 
even when the rest of the mouth is shut. 
Fig. 55.—TRANSVERSE SECTION THROUGH THE UPPER PART OF 
A Younc ACTINIAN. 
Magnified. (After HerTwic and others.) 
Directive Mesenteries. 
Secondary 
Mesenteries. 
“AAR yeqdas-1ajuy 
Ectoderm of = 4 
Gullet. g & 
g2 
gs 
meted 
Gullet. “B 
et 
Ectoderm. Og 
Mesoglcea. 58 
OSS 7 
Endoderm. Zee e 
Secondary Mesenteries. Siphonoglyph. 
Hence Actinia is not truly axo-symmetric like Hydra, 
but is symmetric about two perpendicular planes, the one 
parallel to the long axis of the mouth, the other 
at right angles to it. This comparatively rare 
form of symmetry is called d-plano-symmetry. 
The interior of Actinia yields still more striking 
differences. A transverse section through the 
lower part shows that the internal cavity or 
coelenteron is not simple like that of Hydra, but 
is partially divided into a central gastric cavity and a 
Symmetry. 
Internal 
Features, 
