44 NATURAL HISTORY. [UPPER FLOOR. 
rows of pores, are called Diadems. These often have 
tubular spines, as the common Diadem {Echinus diademci). 
Several of the species of the orbicular kind live in holes 
in rocks, and are believed, by some authors, to have the 
faculty, like the Piddock ( Pholas), of boring into their 
substance. 
The Star-Fish {Asterice) (Cases 9—16) have the body 
depressed and more or less divided into rays, and the sto¬ 
mach furnished with only a single aperture. They have 
been separated into several groups, the first containing 
those with a small orbicular body and long, subcylindrical 
arms, as the Medusa’s Heads {Euryale), which have the 
arms very long, with complex ramifications, so as to end in 
an immense multitude of small threads. In most of the 
species the arms are branched at the base, but in one 
{Euryale palmifera) the base of the arms is simple, and 
the tip repeatedly ramified. 
The Ophiurae have also very long and slender arms, but 
they are always simple. In many of the species the arms 
are furnished on each side with several series of minute, 
moveable spines. 
The arms of the true Star-Fish {Asterias') are a mere 
extension of the substance of the body, and of an uniform 
structure with it. Most of the species have the faculty of 
reproducing the arms, or such parts of them as may be ac¬ 
cidentally broken off; and if an entire arm be separated, 
provided a part of the body be attached to it, other arms 
are reproduced, and a fresh, perfect animal is formed. 
Some specimens illustrative of these facts are in the 
Case 10. 
The Asteriae differ greatly from one another both in 
texture and form. Most of the species have five rays, but 
varieties are sometimes met with which have only four rays, 
one of which is in Case 17. Some species have eight, 
others nine, and others again from twelve to thirty rays. 
The Comatulae (Case 16) have their arms fringed on 
each side with a series of simple rays, and the under part 
of the body furnished with a tuft of simple indexed fibres, 
ending in an incurved hook, by which they attach them¬ 
selves to sea-weeds and other marine bodies. 
One species of Comatula is found on the English coast; 
the largest {Comatula glacialis) is from the Arctic Seas. 
The Fringed Comsituh {Comatula Jimhriata) is from India. 
