96 
LIFE, IN ITS LOWER FORMS. 
But between the Feather-star and the Sea-urchin there 
is so great a diversity in form and appearance, that our 
readers may be reluctant to admit them into the same 
category; the gulf that separates them seems to be 
too wide. Let us see however, if we cannot bridge it 
over. 
If you have ever thrown a dredge overboard in any of 
our quiet bays a mile or two from shore, and examined 
the host of curious and strange things which it has brought 
up from the floor of the sea, you know what a Brittle-star 
is. But if not, we will try to describe it for you. We 
select the commonest species , — Ophiocoma rosula, the 
Rosette. Imagine a central disk about as large as a silver 
fourponny-piece, of a form between a circle and a pentagon, 
composed of five pairs of triangular plates pointing towards 
the centre, and separated by bands of leathery skin stud- 
ded with minute spines. In the centre of the under 
surface of tho disk is the mouth, an aperturo into which 
project five complex plates, and from whose margin spiing 
five long slender rays diverging on overy side, and looking 
much like the tails of so many scaly lizards. 
Now these rays are of exquisite workmanship. They 
appear to be nearly solid columns, with narrow tubular 
canals running through them ; but they are penetrated 
by various organs, with muscles for motion, with glands 
for secretion, with nerves for sensation, and so forth. Ex- 
ternally they are seen to be built up of plates, which fit 
and partly overlap one another, so as to allow great free- 
dom of motion. Those on the upper side are triangular, 
with blunt points; those beneath are square, with tho 
corners cut off. These two series are connected by lateral 
