64 ZOOLOGY 
the mesentery. There are no special sense organs. Nemato- 
cysts occur, and are more complicated than those usually 
found in the Hydrozoa. 
The most fundamental distinction between the Actinozoa 
and the Hydrozoa lies in the presence of the mesenteries and 
the consequent division of the coelenteron into a central 
chamber and several radiating lateral chambers, which all open 
below into the central chamber. In the Hexactinia these 
chambers are put into direct communication with each other 
by one or two pores, which pierce the mesenteries near their 
upper end. The existence of these mesenteries is to some extent 
foreshadowed by the taeniolae of the Scyphomedusae. In two 
forms recently described by Danielssen, Fenja and Aegir, the 
oesophagus is continued to the aboral disk, where it opens to 
the exterior by an anus; thus a body cavity is shut off from 
the digestive cavity. In Aegir, however, these two cavities 
communicate by a series of pores which lead from the inter- 
mesenteric chambers into the alimentary canal near the anus. 
In Fenja each intermesenteric chamber opens to the exterior 
by a pore close to the anus. Thus a ring of genital pores is 
formed. In both genera the various intermesenteric cavities 
communicate with one another by pores at the oral end of the 
mesentery. 
The Actinozoa have an apparent radial symmetry which 
does not hold true for all parts; but a genuine bilateral 
symmetry exists. 
The Actinozoa are sub-divided into two orders: (i.) the 
HEXACTINIA (ZOANTHARIA) and (ii.) the OcTacTiINIA (ALCY- 
ONARIA). 
Order 1. HEXACTINIA. 
CHARACTERISTICS.— Tentacles simple; they and the mesenteries 
are very generally some multiple of six. Single or colonial ; 
when colonial a continuous organic or calcareous skeleton is 
usually deposited by the ectoderm. 
Actinia mesembryanthemum is a beautiful red sea-anemone 
common in rock pools round our coasts. When in the 
expanded condition it may be seen that the animal is in the 
form of a short cylinder, with one end firmly attached to a stone 
