172 THE OCEAN WORLD. 
They are habitually simple, and, if they present ramifications, these 
are gnly exceptional. In nearly every instance, the tentacles exist to 
the number of twelve, eighteen, twenty-four, and even larger numbers, 
and they form a sort of concentric crown to the animal. 
Zoanthus thalassanthos (Lesson), which has given its name to the 
group, consists of large turf-like tufts of coral attached to a rock. 
Its polyps are packed close together, and their expanded flower-like 
heads have a curious resemblance to a mass of flowers in full bloom. 
They are borne on bending root-like stems of pure white, interlacing 
one with the other, surmounted by a fusiform or spindle-shaped body, 
pediculate and swelling towards the middle, but truncate at the 
summit, of a reddish-brown colour, marked with longitudinal stripes 
more highly coloured ; its consistence is firm and parchment-like. 
From the body issues a tube, narrow, muscular, contractile, and red 
in colour, terminating at the summit in eight elongated arms or 
tentacula, of a pure yellow, traversed by a nervure of the same colour. 
The edges of these arms are fringed with fine pinnee, parallel to each 
other, of a bright maroon colour, and resembling the barbs of a 
feather. According to Lesson, the arms of this Zoanthus are kept 
unceasingly in motion, thus producing in the water small oscillating 
currents, in the course of which the animalcules on which the polyps 
feed are precipitated into the stream leading to their mouths. 
The tendency to produce a calcareous or horny polypidom is a 
property almost universal with animals of this class. Zoologists are 
agreed in dividing them into three very distinct orders—namely, the 
ANTIPATHIDA, consisting of the genera Antipathes, Cirripathes, and 
Leipathes, in which the polypidom is of a horny consistence ; the 
MADREPORIDA, in which the polypidom is calcareous and stony; 
finally, the AcTINIDA, which produce no true polypidom. 
ANTIPATHIDA. 
We need not dwell upon this group, which is comparatively unin- 
teresting. They somewhat correspond with the family of Gorgonide 
among the A/yonaria, which they resemble in having the central axes 
branching after the manner of a shrub; but the polyps have the 
mouth surrounded with a crown of six simple tentacula. The axis 
is of a harder and denser tissue than that of the Gorgonide, and 
presents on its surface small spiniform projections. The polypiferous 
crust, with which they are covered, is in general very arenaceous, 
and is so easily detached, that it is rare to see in collections anything 
but the denuded skeleton of the colony. In Axtipathes arborea, the 
