30 ZOOPHYTES. 
related species, which are the types of other divisions of the order. 
The compound structure, the mode of growth, and the formation of 
the corallum, in the several groups, will come next under consi- 
deration. 
Il. GENERAL STRUCTURE OF THE ACTINOIDEA. 
20. The polyps of the Actinoidea correspond well with the cha- 
racter drawn on a preceding page. A circular disk, fringed with 
tentacles, —in shape much like an Aster with its petals, —and 
having a mouth at its centre, forms the upper part or extremity of 
the polyp. The mouth opens through a nearly cylindrical stomach 
into a large visceral cavity closed at bottom. The mouth receives 
the food and also gives exit to what remains after digestion. 
The Actinia. 
21. The Actinia is commonly met with attached by its flat under 
surface to rocks along the sea-shores. When unexpanded, it looks 
like a rounded lump of animal matter, somewhat leathery in ap- 
pearance, plastered on the rock; it shows nothing of the mouth, 
and none of the fringing tentacles, these being concealed by the 
involuted margin of the summit. As the animal expands, the central 
opening at the top gradually widens,—the margin slowly rolls 
Actinia. 
back, and the tentacles it concealed now begin to show their tips. 
As the expansion goes on, the tentacles continue to enlarge, and the 
margin to spread outward, till finally the disk with the mouth at 
centre, is laid open, and the tentacles, like petals, fringe it around. 
Such is the general appearance of an Actinia, and such also are the 
greater part of coral polyps, which are nothing but Actinias, possess- 
