NOTES ON THE OPHIURANS, OH THE SAND AND BRITTLE STARS. 341 
on its back the under part of the body presents itself, and its 
middle is very pretty-looking, and very geometrical, from the 
quintuple repetition of regularly-shaped plates converging to 
the central mouth. Between the elements of this quinary 
arrangement are the lower parts of the arms, which come well 
within the body like so many radii, and reach to the slits which 
surround the mouth. Each arm in its radial course confines a 
portion of the body between it and its neighbour, and the 
space is limited, without by the edge of the body (the margin), 
and within by some plates which point to the mouth. This 
space is an interbrachial space, and on it, close to the arms, are 
slits or linear openings, one on either side usually, but in some 
kinds two. These lead upwards, in the proper position of the 
animal, into cavities — the generatives. Sometimes the inter- 
brachial spaces are naked, but in most genera, they are covered 
with a less regular and smaller plating or scaling or spinuling than 
the upper part of the body. At the inner angle of each inter- 
brachial space is a plate, one of which is sometimes porous, called 
the mouth shield (Figs. 2, 4, and 10 b). Within it, that is to 
say, nearer the centre, or, as the term is, “ orally,” are two 
smaller plates (Figs. 4, 9, 10 c) called side-mouth shields, 
and orally or internally to them is a more or less tri- 
angular mass, fringed at the free sides and tip, (Figs. 4, 9 d) 
with a few or many papillae. Careful examination will show 
that the triangular surface is divided by a line running nearly 
from the apex to the base, and really there are two triangles 
side by side. These are the under surfaces of the jaws, 
and at their apex the innermost papilla is upon a small plate, 
the under part of which is seen from below, and it is called the 
jaw plate. A different view explains that it runs up into the 
body, fixed on to the jaws, and that it supports some projections 
called “true teeth.” There are five of these jaw arrange- 
ments, each of which is called “ an angle,” and the roots, so to 
speak, of the arms fit in between their bases between neighbour- 
ing mouth shields. (Figs. 2, 4, 9 d, jaws ; e, teeth ; e p, jaw 
plate). 
The arms, whether long or short, spiny or nearly plain, are 
composed of an outside of more or less regular plates, and of an 
inside support. They must be tolerably strong, more or less pro- 
tected, able to move, and, oddly enough, capable of being 
separated here and there, and cast off, to be again repaired. 
Their conformation relates to the habits of the animal, and as 
some Ophiurans wriggle th^m much horizontally, and indulge 
in serpentine and interlocking movements wonderful to behold, 
they must have the scales on the arm arranged to permit of 
this, especially as in these instances long spines stick out from 
the sides of the arms. Such arms are of necessity not capable 
