CCELENTERATA. 



the general appearance closely resembles that of a very 

 large but short and broad Hydra, An examination of the 

 mouth, however, will show that it is not circular like that 

 of Hydra 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 Young Actinian. 



Magnified. (After Hertwig and others.) 

 Directive Mesenteries. 



Ectoderm of. 

 Gullet. 



Gullet. 



Ectoderm, 

 Mesogloea. 



Endoderm. 



vSecondary 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 bi-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 



Ssmimetry. 



Internal 

 Features. 



